摘要:

4369 4377 4387 4397 4407 4417 4427 4439 445 1 4457 447 1 4475 4487 4495 450 1 4509 Con tents A New Form of the High-temperature Isopiestic Technique and its Applica- tion to Mercury-Bismuth, Mercury-Cadmium, Mercury-Gallium, Mercury- Indium and Mercury-Tin Binary Amalgams Z-C. Wang, X-H. Zhang, Y-Z. He and Y-H. Bao The Derivation of Chemical-diffusion Coefficients of Oxygen in UO,,, over the range 180-300 "C. Spectroscopic Procedure and Preliminary Results T. R. Griffiths, H. V. St. Aubyn Hubbard, G. C. Allen and P. A. Tempest Pho tophysics at Solid Surfaces. Evidence of Dimer Formation and Polarization of Monomer and Excimer Fluorescences of Pyrene in the Adsorbed State on Silica-gel Surfaces T. Fujii, E. Shimizu and S. Suzuki Ordering in Monodispersed Polymer Latices induced by a Temperature Gradient K.Furusawa, N. Tobori and S. Hachisu X-Ray Diffraction Study of Molten Eutectic LiF-NaF-KF Mixture K. Igarashi, Y. Okamoto, J. Mochinaga and H. Ohno Viscosity Measurements of Some Tetra butylammonium, Copper( I), Silver( I) and Thallium( 1) Salts in Acetonitrile-Pyridine Mixtures at 15, 25 and 35 "C D. S. Gill and B. Singh The Ethane- 1,2-diol-Water Solvent System. The Dependence of the Dis- sociation Constant of Picric Acid on the Temperature and Composition of the Solvent Mixture Silver(1) Complexation with Tertiary Amines in Toluene M. Soledade Santos, E. F. G. Barbosa and M. Spiro Enhanced Oxygen Evolution through Electrochemical Water Oxidation mediated by Polynuclear Complexes embedded in a Polymer Film G. J. Yao, A. Kira and M. Kaneko Nature of Acid Sites in SAP05 Molecular Sieves.Part 1.-Effects of the Concentration of Incorporated Silicon C. Halik, J. A. Lercher and H. Mayer Hemimicelle Formation of Cationic Surfactants at the Silica Gel-Water Interface T. Gu, Y. Gao and L. He Nuclear Magnetic Resonance Relaxation in Micelles. Deuterium Relaxation at Three Field Strengths of Three Positions on the Alkyl Chain of Sodium Dodecyl Sulphate Studies of the Temperature Dependence of Retention in Supercritical Fluid Chromatography K. D. Bartle, A. A. Clifford, J. P. Kithinji and G. F. Shilstone Hydrogen and Muonium Atom Adducts of Trimethylsilyl Derivatives of Ethyne The Radical Cation of Formaldehyde in a Freon Matrix. An Electron Spin Resonance Study Phase Transition of the Water confined in Porous Glass studied by the Spin- probe Method H.Yoshioka G. C. Franchini, A. Marchetti, L. Tassi and G. Tosi 0. Soderman, G. Carlstrom, U. Olsson and T. C. Wong C. J. Rhodes and M. C. R. Symons C. J. Rhodes and M. C. R. Symons4369 4377 4387 4397 4407 4417 4427 4439 445 1 4457 447 1 4475 4487 4495 450 1 4509 Con tents A New Form of the High-temperature Isopiestic Technique and its Applica- tion to Mercury-Bismuth, Mercury-Cadmium, Mercury-Gallium, Mercury- Indium and Mercury-Tin Binary Amalgams Z-C. Wang, X-H. Zhang, Y-Z. He and Y-H. Bao The Derivation of Chemical-diffusion Coefficients of Oxygen in UO,,, over the range 180-300 "C. Spectroscopic Procedure and Preliminary Results T. R. Griffiths, H. V. St. Aubyn Hubbard, G. C. Allen and P. A. Tempest Pho tophysics at Solid Surfaces.Evidence of Dimer Formation and Polarization of Monomer and Excimer Fluorescences of Pyrene in the Adsorbed State on Silica-gel Surfaces T. Fujii, E. Shimizu and S. Suzuki Ordering in Monodispersed Polymer Latices induced by a Temperature Gradient K. Furusawa, N. Tobori and S. Hachisu X-Ray Diffraction Study of Molten Eutectic LiF-NaF-KF Mixture K. Igarashi, Y. Okamoto, J. Mochinaga and H. Ohno Viscosity Measurements of Some Tetra butylammonium, Copper( I), Silver( I) and Thallium( 1) Salts in Acetonitrile-Pyridine Mixtures at 15, 25 and 35 "C D. S. Gill and B. Singh The Ethane- 1,2-diol-Water Solvent System. The Dependence of the Dis- sociation Constant of Picric Acid on the Temperature and Composition of the Solvent Mixture Silver(1) Complexation with Tertiary Amines in Toluene M.Soledade Santos, E. F. G. Barbosa and M. Spiro Enhanced Oxygen Evolution through Electrochemical Water Oxidation mediated by Polynuclear Complexes embedded in a Polymer Film G. J. Yao, A. Kira and M. Kaneko Nature of Acid Sites in SAP05 Molecular Sieves. Part 1.-Effects of the Concentration of Incorporated Silicon C. Halik, J. A. Lercher and H. Mayer Hemimicelle Formation of Cationic Surfactants at the Silica Gel-Water Interface T. Gu, Y. Gao and L. He Nuclear Magnetic Resonance Relaxation in Micelles. Deuterium Relaxation at Three Field Strengths of Three Positions on the Alkyl Chain of Sodium Dodecyl Sulphate Studies of the Temperature Dependence of Retention in Supercritical Fluid Chromatography K. D. Bartle, A. A. Clifford, J. P. Kithinji and G. F. Shilstone Hydrogen and Muonium Atom Adducts of Trimethylsilyl Derivatives of Ethyne The Radical Cation of Formaldehyde in a Freon Matrix. An Electron Spin Resonance Study Phase Transition of the Water confined in Porous Glass studied by the Spin- probe Method H. Yoshioka G. C. Franchini, A. Marchetti, L. Tassi and G. Tosi 0. Soderman, G. Carlstrom, U. Olsson and T. C. Wong C. J. Rhodes and M. C. R. Symons C. J. Rhodes and M. C. R. Symons

ISSN:0300-9599    

DOI:10.1039/F198884FX017

出版商:RSC    

年代:1988

数据来源: RSC

摘要:

NOMENCLATURE AND SYMBOLISM Units and Symbols. The Symbols Committee of The Royal Society, of which The Royal Society of Chemistry is a participating member, has produced a set of recommendations in a pamphlet 'Quantities, Units, and Symbols' (1 975) (copies of this pamphlet and further details can be obtained from the Manager, Journals, The Royal Society of Chemistry, Burlington House, London W1V OBN). These recommendations are applied by The Royal Society of Cemistry in all its publications. Their basis is the 'Systeme International d'Unit6s' (9). A more detailed treatment of units and symbols with specific application to chemistry is given in the IUPAC Manual of Symbols and Terminology for Physicochemical Quantities and Units (Pergamon, Oxford, 1979). Nomenclature. For many years the Society has actively encouraged the use of standard IUPAC nomenclature and symbolism in its publications as an aid to the accurate and unambiguous communication of chemical information between authors and readers.In order to encourage authors to use IUPAC nomenclature rules when drafting papers, attention is drawn to the following publications in which both the rules themselves and guidance on their use are given: Nomenclature of Organic Chemistry, Sections A, B, C, D, E, F, and H (Pergamon, Oxford, 1979 edn). Nomenclature of Inorganic Chemistry (Butterworths, London, 1971 , now published by Pergamon). Biochemical Nomenclature and Related Documents (The Biochemical Society, London, 1978). Compendium of Chemical Terminology: IUPAC Recommendations (Blackwells, Oxford, 1987).A complete listing of all IUPAC nomenclature publications appears in the January issues of J. Chem. SOC., Faraday Transactions. It is recommended that where there are no IUPAC rules for the naming of particular compounds or authors find difficulty in applying the existing rules, they should seek the advice of the Society's editorial staff. (xiv)NOMENCLATURE AND SYMBOLISM Units and Symbols. The Symbols Committee of The Royal Society, of which The Royal Society of Chemistry is a participating member, has produced a set of recommendations in a pamphlet 'Quantities, Units, and Symbols' (1 975) (copies of this pamphlet and further details can be obtained from the Manager, Journals, The Royal Society of Chemistry, Burlington House, London W1V OBN). These recommendations are applied by The Royal Society of Cemistry in all its publications.Their basis is the 'Systeme International d'Unit6s' (9). A more detailed treatment of units and symbols with specific application to chemistry is given in the IUPAC Manual of Symbols and Terminology for Physicochemical Quantities and Units (Pergamon, Oxford, 1979). Nomenclature. For many years the Society has actively encouraged the use of standard IUPAC nomenclature and symbolism in its publications as an aid to the accurate and unambiguous communication of chemical information between authors and readers. In order to encourage authors to use IUPAC nomenclature rules when drafting papers, attention is drawn to the following publications in which both the rules themselves and guidance on their use are given: Nomenclature of Organic Chemistry, Sections A, B, C, D, E, F, and H (Pergamon, Oxford, 1979 edn). Nomenclature of Inorganic Chemistry (Butterworths, London, 1971 , now published by Pergamon). Biochemical Nomenclature and Related Documents (The Biochemical Society, London, 1978). Compendium of Chemical Terminology: IUPAC Recommendations (Blackwells, Oxford, 1987). A complete listing of all IUPAC nomenclature publications appears in the January issues of J. Chem. SOC., Faraday Transactions. It is recommended that where there are no IUPAC rules for the naming of particular compounds or authors find difficulty in applying the existing rules, they should seek the advice of the Society's editorial staff. (xiv)

ISSN:0300-9599    

DOI:10.1039/F198884BX019

出版商:RSC    

年代:1988

数据来源: RSC

摘要:

ISSN 0300-9599 JCFTAR 84(5) 1287-1 749 (1 988) JOURNAL OF THE CHEMICAL SOCIETY Faraday Transactions I Physical Chemistry in Condensed Phases Quantities, Units and Symbols The Faraday Society had an idiosyncratic approach to the use of symbols in physical chemistry and, it must be admitted, some little quirks survive to this day. Thus we continue to italicise symbols for atomic term states S, P, D etc. and certain important constants are still printed in bold-face italic, k, F, R and N. These oddities are mostly tolerated, though I do treasure a letter from a Dutch professor in which it is explained that Boltzmann's constant is not a vector. For the future, we must try to reform, but echo the plea of St. Augustine by doing it gradually. To this end we will continue to consult "Quantities, Units and Symbols" published by The Royal Society, but add to our armoury a recent IUPAC publication prepared under the general direction of Professor Ian M.Mills of Reading University. This is: "Quantities, Units and Symbols in Physical Chemistry", IU PAC and Blackwell Scientific Pub- lications, Oxford, 1988. It goes further than the Royal Society booklet in several respects and I commend it to all our authors. D. A. Young 1287 1301 131 1 1329 1341 1349 1357 1375 1387 1393 1405 43 CONTENTS Photophysical and Photochemical Properties of CdS with Limited Dimensions R. D. Stramel, T. Nakamura and J. K. Thomas Precipitation of Calcium Oxalates from High-ionic-strength Solutions. Part 6.-Kinetics of Precipitation from Solutions Supersaturated in Calcium Oxalates and Phosphates Hydrocarbon Formation from Methanol and Dimethyl Ether using WO,/AI,O, and H-ZSM-5 Catalysts.A Mechanistic Investigation using Model Reagents G. J. Hutchings, L. J. van Rensburg, W. Pick1 and R. Hunter A New Ti0,-attached Rhodium Metal Catalyst, Catalyst Characterization and Non-SMSI Behaviour Hydrophilic and Hydrophobic Phenomena in Electrolyte Solutions C-F. Pan Thermal Decomposition of Copper(r1) Squarate A. K. Galwey, M. A-A. Mohamed, S. Rajam and M. E. Brown The Thermal Decomposition Reactions of Copper(I1) Maleate and of Copper(1r) Fumarate Influence of Bound Water on Viscoelastic Enhancement in Aqueous Disperse Systems of Ionic Microgels Interpretation of Free Energies of Transfer and Solute Partial Molar Volumes in Mixed Solvents using Kirkwood-Buff Theory Application of Kirkwood-Buff Theory to Free Energies of Transfer of Electrolytes from One Solvent to Another A.K. Covington and K. E. Newman Structure and Reactivity of Zinc-Chromium Mixed Oxides. Part 1 .-The Role of Non-stoichiometry on Bulk and Surface Properties M. Bertoldi, B. Fubini, E. Giamello, G. Busca, F. Trifiro and A. Vaccari M. Markovik and H. Fiiredi-Milhofer K. Asakura, Y. Iwasawa and H. Kuroda N. J. Carr and A. K. Galwey T. Matsumoto, D. Ito and S. Yao K. E. Newman FAR 1Con tents Structure and Reactivity of Zinc-Chromium Mixed Oxides. Part 2.-Study of the Surface Reactivity by Temperature-programmed Desorption of Methanol A. Riva, F. Trifiro, A. Vaccari, L. Mintchev and G. Busca Excess Enthalpies and Excess Volumes of (OSCO, + 0.5C2H,) in the Super- critical Region Correlations between Wavenumbers of Skeletal Vibrations, Unit-cell Size and Molar Fraction of Aluminium of Y Zeolites. Removal of Non-skeletal A1 Species with H,Na,EDTA L.Kubelkova, V. Seidl, G. Borbdy and H. K. Beyer Preparation of Model Mesoporous Carbons Preferential Solvation of Ions in Mixed Solvents. Part 2.-The Solvent Composition near the Ion Electron Spin Resonance Study of the Copper(ii) and Cobalt(I1) Chelates of 2,3 ; 7,8 ; 12,13 ; 17,18-Tetrakis-(9,1O-dihydroanthracene-9,10-diyl)porphyra- zine S. W. Oliver, T. D. Smith, G. R. Hanson, N. Lahy, J. R. Pilbrow and G. R. Sinclair The Influence of Water on the Oxygen-Silver Interaction and on the Oxidative Dehydrogenation of Methanol L. Lefferts, J.G. van Ommen and J. R. H. Ross The Charge- transfer Band of N-Alkylpyridinium Iodides in Mixed Aqueous Solvents K. Medda, M. Pal and S. Bagchi Enhancement of Lewis Acidity in Layer Aluminosilicates. Treatment with Acetic Acid An Electrostatic Approach to the Structure of Hydrated Lanthanoid Ions. [M(OH,)J3+ cersus [M(OH2),l3+ Chiral Sulphur-containing Molecules in Langmuir-Blodgett Films C. Georges, T. J. Lewis, J. P. Llewellyn, S. Salvagno, D. M. Taylor, C. J. M. Stirling and V. Vogel Stochastic Interpretation of Lag Times for the Onset of Template Amplification in RNA Replication A. Fernandez Coordination States of Divalent Transition-metal Perchlorates in Hexa- methylphosphoramide Solutions W. Grzybkowski, M. Pilarczyk and L. Klinszporn Scattering from Polyelectrolyte Solutions M.J. Grimson, M. Benmouna and H. Benoit Third-body Tnteractions in the Oscillatory Oxidation of Hydrogen in a Well Stirred Flow Reactor D. L. Baulch, J. F. Griffiths, A. J. Pappin and A. F. Sykes Gas Sensitivity of Polypyrrole Films to NO, T. Hanawa, S. Kuwabata and H. Yoneyama Determination of the Surface Coverage of Oxidic Supports by Oxidic and Non- oxidic Supported Phases using Potentiometric Titration and Electrophoretic Mobility Data. A Study of Fe,O,/Al,O, Supported Catalysts L. Vordonis, C. Kordulis and A. Lycourghiotis Liquid Structure and Second-order Mixing Functions for 1 -Chloronaphthalene with Linear and Branched Alkanes M. Costas, H. Van Tra, D. Patterson, M. Caceres- Alonso, G. Tardajos and Emilio Aicart An Infrared Spectroscopic Study of Anatase Properties.Part 6.-Surface Hydration and Strong Lewis Acidity of Pure and Sulphate-doped Preparations C. Morterra Raman Spectra of Potassium trans-4-Hexenoate and Conformational Change on Micellization H. Okabayashi, K. Tsukamoto, K. Ohshima, K. Taga and E. Nishio C. J. Wormald and J. M. Eyears D. H. Everett and F. Rojas Y. Marcus C. R. Theocharis, K. J. $Jacob and A. C. Gray K. Miyakawa, Y. Kaizu and H. Kobayashi 1423 1437 447 45 5 465 1475 1491 1501 1509 1517 1531 1543 1551 1563 1575 1587 1593 1603 1617 1639Contents 1653 1671 1679 1691 1703 1713 1723 1729 1737 1741 Solutions of Organic Solutes. Part 3.-Compression and Structure J. V. Leyendekkers Thermodynamic Parameters for the Transfer of Electrolytes and Ions from Water to Benzonitrile Basicity of Water in Organic Solvents (Nitromethane, Propane- l12-diyl- Carbonate and Acetone) using the 1,4,8,ll-Tetramethyl- 1,4,8,1 l-tetra-aza- cyclotetradecanenickel( 11) Cation as a Probe of Electron-pair Acceptance S.Yamasaki, E. Iwamoto and T. Kumamaru Oxidation of Chloride to Chlorine by Cerium(1v) Ions Mediated by a Microheterogeneous Redox Catalyst A. Mills and A. Cook Preparation of SrTiO, by Sol-Gel Techniques for the Photoinduced Production of H, and Surface Peroxides from Water K. R. Thampi, M. Subba Rao, W. Schwarz, M. Gratzel and J. Kiwi Interactions between Metal Cations and the Ionophore Lasalocid. Part 4.- AHg and ASg for Formation of 1-1 and 2-1 Complexes of the Lasalocid Anion and Salicylate with Alkaline-earth Metal Cations in Methanol Y.Pointud, E. Passelaigue and J. Juillard An Electron Spin Resonance Study of the Superoxide Radical Anion in Poly- crystalline Magnesium Oxide and Titanium Dioxide Powders A. Amorelli, J. C. Evans and C. C. Rowlands Transference Number Measurements of Copper(1) Perchlorate in Binary Mixtures of Acetonitrile with Water, Methanol, Acetone and N,N-Dimethyl- formamide at 25 "C The Elimination of Internal and External References in Nuclear Magnetic Resonance Determinations of Fast Equilibria with Particular Reference to Electron-donor-Acceptor (EDA) Complex Formation J. A. Chudek, R. Foster and R. L. MacKay Carbon Monoxide Hydrogenation over Silica-supported Rhodium Catalysts. The Effect of the Rhodium Precursor S. D. Jackson, B. J. Brandreth and D. Winstanley A.F. Danil de Namor and H. Berroa de Ponce D. S. Gill, K. S. Arora, J. Tewari and B. Singh 43-2Contents 1653 1671 1679 1691 1703 1713 1723 1729 1737 1741 Solutions of Organic Solutes. Part 3.-Compression and Structure J. V. Leyendekkers Thermodynamic Parameters for the Transfer of Electrolytes and Ions from Water to Benzonitrile Basicity of Water in Organic Solvents (Nitromethane, Propane- l12-diyl- Carbonate and Acetone) using the 1,4,8,ll-Tetramethyl- 1,4,8,1 l-tetra-aza- cyclotetradecanenickel( 11) Cation as a Probe of Electron-pair Acceptance S. Yamasaki, E. Iwamoto and T. Kumamaru Oxidation of Chloride to Chlorine by Cerium(1v) Ions Mediated by a Microheterogeneous Redox Catalyst A. Mills and A. Cook Preparation of SrTiO, by Sol-Gel Techniques for the Photoinduced Production of H, and Surface Peroxides from Water K.R. Thampi, M. Subba Rao, W. Schwarz, M. Gratzel and J. Kiwi Interactions between Metal Cations and the Ionophore Lasalocid. Part 4.- AHg and ASg for Formation of 1-1 and 2-1 Complexes of the Lasalocid Anion and Salicylate with Alkaline-earth Metal Cations in Methanol Y. Pointud, E. Passelaigue and J. Juillard An Electron Spin Resonance Study of the Superoxide Radical Anion in Poly- crystalline Magnesium Oxide and Titanium Dioxide Powders A. Amorelli, J. C. Evans and C. C. Rowlands Transference Number Measurements of Copper(1) Perchlorate in Binary Mixtures of Acetonitrile with Water, Methanol, Acetone and N,N-Dimethyl- formamide at 25 "C The Elimination of Internal and External References in Nuclear Magnetic Resonance Determinations of Fast Equilibria with Particular Reference to Electron-donor-Acceptor (EDA) Complex Formation J. A. Chudek, R. Foster and R. L. MacKay Carbon Monoxide Hydrogenation over Silica-supported Rhodium Catalysts. The Effect of the Rhodium Precursor S. D. Jackson, B. J. Brandreth and D. Winstanley A. F. Danil de Namor and H. Berroa de Ponce D. S. Gill, K. S. Arora, J. Tewari and B. Singh 43-2

ISSN:0300-9599    

DOI:10.1039/F198884FP059

出版商:RSC    

年代:1988

数据来源: RSC

摘要:

JOURNAL OF THE CHEMICAL SOCIETY Faraday Transactions 11, Issue 5,1988 Molecular and Chemical Physics Dr Gus Hancock of Oxford University was invited to contribute a Keynote Paper on the general theme of Laser Studies of Gas-phase Kinetics and Photochemistry. He was supported by a group of research workers who submitted original papers on cognate subjects. All these papers have now been refereed, and are collected in this month’s issue of Faraday Transactions II. For the benefit readers of Faraday Transactions I, the contents list is reproduced below. 429 Laser Studies of Gas-phase Kinetics and Photochemistry G. Hancock 441 Elementary Deactivation Processes of CHF[A”’ A”(0,O < u, 5, O)] after Single Vibronic Level Excitation by He, Ar and the CHF Precursor CH,F, G. Dornhofer and W.Hack 455 Spectroscopic Identification of C(,P) Atoms in Halogenomethane + H Flame Systems and Measurements of C(’P) Reaction Rate Constants by Two- photon Laser-induced Fluorescence K. H. Becker, K. J. Brockmann and P. Wiesen The Spectroscopy, Photophysics and Photodissociation Dynamics of Jet- cooled CF,NO [z(n, n*)] J. A. Dyet, M. R. s. McCoustra and J. Pfab The 248 nm KrF Laser Excitation of Alkyl Iodide-Fluorine Mixtures. The Production and Spectroscopy of CF,(A”) D. Raybone, T. M. Watkinson and J. C. Whitehead Kinetics of the Reaction between Oxygen Atoms and Ethyl Radicals I. R. Slagle, D. Sarzynski, D. Gutman, J. A. Miller and C. F. Melius A Global Technique for Analysing Multiple Decay Curves. Application to the CH, + 0, System M. Keiffer, A.J. Miscampbell and M. J. Pilling Absolute Rate Measurements for some Gas-phase Addition Reactions of Dimethylsilene J. E. Baggot, M. A. Blitz, J. M. Frey, P. D. Lightfoot and R. Walsh 527 Pulsed Laser Photolysis - Laser-induced Fluorescence Measurements on the Kinetics of CN(u = 0) and CN(u = 1) with 0,, NH and NO between 294 and 761 K I. R. Sims and I. W. M. Smith Optical-Optical Double Resonance (OODR) Studies of the Ion-pair States of the Halogens. Part 1 .-Vibronic Threshold for the Chemiluminescent Reaction between I,(f 0,’) and Xe R. J. Donovan, A. J.-Holmes, P. R. R. Langridge-Smith and T. Ridley A Laser Photolysis/LIF Study of the Reactions of O(,P) Atoms with CH, and CH,O Radicals R. Zellner, D. Hartman, J. Karthauser, R. Rhasa and G. Weibring 569 Photofragment Vector Correlations and Dissociation Dynamics in HONO, F.Caralp, R. Lesclaux, M. Lesclaux, M. T. Rayez, J. C. Rayez and W. Forst 463 483 491 505 5 15 54 1 549 (0587 Photofragmented Vector Correlation Reactions of Chlorofluoromethylperoxy Radicals with NO, in the Temperature Range 233-373 K J. August, M. Brouard and J. P. Simons The following papers were accepted for publication in Faraday Transactions I during February, 1988. Activity Measurements and Spectroscopic Studies on the Catalytic Oxidation of Toluene over Vanadium Oxides supported on Silica/Alumina B. Johnson, B. Rebenstorf, R. Larsson and S. L. T. Anderson 71 1707 Bronsted Relations for Heterogeneous Proton Transfer at Electrode Interfaces B. E. Conway and D. P. Wilkinson 7/ 181 1 An Investigation of Structural Changes in the Nickel-Uranium Oxide Catalyst System by Uranium L,-Edge and Nickel K-Edge EXAFS and XANES 7/ 1835 Thermal Decomposition of Pyrite : Kinetic Analysis of Thermogravimetric Data by Predictor-Corrector Numerical Methods I.C. Hoare, H. J. Hurst, W. I. Stuart and T. J. White 7/1855 Thermal Decomposition of KIO, in Relation to Solid-state Isotopic Exchange Reactions S. Takriti and G. Duplatre 7/1950 Aspects of the Oxidation of Naphthazarin as studied by Pulse Radiolysis T. Mukherjee, E. J. Land and J. A. Swallow 7/2011 The Enthalpies of Interaction of some Alkali Metal Halide Salts with Formamide in Water at 25 "C P. J. Cheek, M. Asuncion Gallado-Jimenez and T. H. Lilley 7/2047 On the Interpretation of X.P. Spectra of Defective Nickel Oxide M.Tomellini 7/2049 Spectroscopic Studies of the Solvation of N,N-Dimethyl Amides in Pure and Mixed Solvents 7/2069 Micellar Effect on the Photosensitized Debromination of 2,3-Dibromo-3- phenylpropionic Acid. Control of Forward and Back Electron Transfers K. Takagi, N. Miyake, E. Nakamura, H. Usami, Y. Sawaki and H. Iwamura Local Motions of Counterions in Polyelectrolyte Solutions without Added Salts studied by Neutron Quasielastic Scattering T. Kanaya, K. Kaji, R. Kitamaru, B. Gabrys and J. S. Higgins 7/2086 Proton N.M.R. Study of Solute-Solvent Interaction of 1 -Methyl-2-pyrrolidone and some Substituted Benzenes H. A. Zainel, S. F. Al-Azzawi and H. I. Swellem 7/2121 Acidic Properties of Vanadium Oxide on Titania H. Miyata, K. Fujii and T. Ono 7/2165 The Influence of Organic Solutes on the Self-diffusion of Water as studied by N.M.R.7/2220 Nature of the Beta Phase of Bismuth Molybdate M. El Jamel, M. Forissier and A. Auroux 7/228 1 Dielectric Constant and Pyroelectricity for the Compounds (CH,),(NH,), FeCl,_,Br, (x = 0 and I ) 7/ 1649 F. J. Berry, A. Murrey and A. T. Steel G. Eaton and M. C. R. Symons 7/207 1 P. 0. Eriksson, E. E. Burnell, G. J. T. Tiddy and G. Lindblom M. A. Ahmed, M. A. Mousa and M. M. Eldesoky (ii)7/2283 Activity Measurements and Spectroscopic Studies of the Catalytic Oxidation of Toluene over Vanadium Oxides supported on Titania B. Jonson, B. Rebenstorf, R. Larsson and S. L. T. Anderson 8/024 Solubilities and Vapour Pressures in the Quinquinary System NaC1,- KCl-MgC1,-H,O Part 1 .-Predictions and Measurements at 25 "C Y.Marcus and N. Soffer Theoretical Interpretation of the Heats of Immersion of Lower n-Alcohols on Faujasites and Pentasiles W. D. Einicke, U. Messow, R. Schollner and G. Zahn 8/052 Measurements of Tracer Diffusion Coefficients of Sulphate Ions in Aqueous Solutions of Ammonium Sulphate and Sodium Sulphate, and Water in Aqueous Sodium Sulphate Solutions K. Tanaka Highly Resolved Solid-state Proton Magnetic Resonance Studies of Zeolites H. Pfeifer 8/05 1 8/ 1 15 (iii)Cumulative Author Index 1988 Abe, H., 511 Abraham, M. H., 175, 865 Adachi, H., 1091 Aicart, E., 1603 Allen, G. C., 165, 355 Amorelli, A., 1723 Anazawa, I., 275 Anpo, M., 751 Aracil, J., 539 Arora, K. S., 1729 Asakura, K., 1329 Aveyard, R., 675 Baba, K., 459 Bagchi, S., 1501 Baglioni, P., 467 Baldini, G., 979 Barna, T., 229 Baulch, D.L., 1575 Bazsa, G., 215, 229 Benmouna, M., 1563 Benoit, H., 1563 Berei, K., 367 Berroa de Ponce, H., 255, 1671 Bertoldi, M., 1405 Beyer, H. K., 1447 Binks, B. P., 675 Blandamer, M. J., 1243 Blesa, M. A., 9 Blinov, N. N., 1075 Bonnefoy, J., 941 Borbely, G., 1447 Borckmans, P., 1013 Borgarello, E., 261 Bourdillon, C., 941 Brandreth, B. J., 1741 Breen, J., 293 Briggs, B., 1243 Brown, M. E., 57, 1349 Brydson, R., 617, 631 Burgess, J., 1243 Burget, U., 885 Busca, G., 237, 1405, 1423 Buxton, G. V., 1101, 11 13 Caceres, M., 539 Caceres-Alonso, M., 1603 Carbone, A. I., 207 Carr, N. J., 1357 Cavani, F., 237 Cavasino, F. P., 207 Centi, G., 237 Chagas, A. P., 1065 Chandra, H., 609 Che, M., 751 Cheng, V.K. W., 899 Chien, J. C. W., 1123 Chirico, G., 979 Chudek, J. A., 1145, 1737 Clarke, R. J., 365 Clint, J. H., 675 Coates, J. H., 365 Coller, B. A. W., 899 Coluccia, S., 751 Compton, R. G., 473, 483 Cook, A., 1691 Costas, M., 1603 Covington, A. K., 1393 Crowther, N. J., 121 1 Danil de Namor, A. F., 255, Das, S., 1057 Dash, A. C., 75 Dash, N., 75 Davydov, A., 37 Dawber, J. C., 41 Dawber, J. G., 41, 713 de Bleijser, J., 293 Diaz Peiia, M., 539 Dickinson, E., 871 Disdier, J., 261 Domen, K., 511 Dougal, J. C., 657 Duarte, M. Y., 97, 367 Duce, P. P., 865 Duckworth, R. M., 1223 Dyster, S., 11 13 Eagland, D., 121 1 Egawa, C., 321 Einfeldt, J., 93 1 Elliot, A. J., 1101 Engel, W., 617, 631 Eszterle, M., 575 Evans, J. C., 1723 Everett, D. H., 1455 Eyears, J. M., 1437 Fernandez, A., 1543 Fernandez-Pineda, C., 647 Flanagan, T. B., 459 Fletcher, P.D. I., 1131 Foresti, E., 237 Foresti, M. L., 97 Forster, H., 491 Foster, R., 1145, 1737 Franklin, K. R., 687 Fubini, B., 1405 Furedi-Milhofer, H., 1301 Gal, D., 1075 Gabrail, S., 41 Galwey, A. K., 57, 729, 1349, Gans, P., 657 Geblewicz, G., 561 Geertsen, S., 1101 1671 1357 Georges, V., 1531 Giamello, E., 1405 Gill, D. S., 1729 Gill, J. B., 657 Gilot, B., 801 Gopalakrishnan, R., 365 Grampp, G., 366 Gratzel, M., 197, 1703 Gray, A. C., 1509 Gray, P., 993 Green, S. I. E., 41 Griffiths, J. F., 1575 Grigo, M., 931 Grimson, M. J., 1563 Gritzner, G., 1047 Grzybkowski, W., 1551 Guardado, P., 1243 Guarini, G. G. T., 331 Guidelli, R., 97, 367 Gupta, D. Das, 1057 Hadjiivanov, K., 37 Hall, D.G., 773 Halle, B., 1033 Hamada, K., 1267 Hanawa, T., 1587 Hanson, G. R., 1475 Harrer, W., 366 Hasebe, T., 187 Hashimoto, K., 87 Hazra, D. K., 1057 Heatley, F., 343 Herley, P. J., 729 Herrmann, J-M., 261 Heyward, M. P., 815 Hidalgo, M. del V., 9 Hill, A., 255 Hubbard, C. D., 1243 Huis, D., 293 Hunter, R., 131 1 Hutchings, G. J., 1311 Ige, J., 1 Ikeda, S., 151 Imai, H., 923 Imamura, H., 765 Imanaka, T., 851 Inoue, A., 1195 Irinyi, G., 1075 Isobe, T., 1199 Ito, D., 1375 Iwamoto, E., 1679 Iwasawa, Y., 321, 1329 Jackson, S. D., 1741 Jaenicke, W., 366 Jeminet, G., 951 Johnson, G. R. A., 501 Johnson, I., 551Johnston, C., 309 Jorge, R. A., 1065 Jorgensen, N., 309 Juillard, J., 951, 959, 969, 1713 Kaizu, Y., 1517 Kane, H., 851 Kanno, T., 281 Kasahara, S., 765 Kato, S., 151 Katz, N.E., 9 Kawasaki, Y., 1083 Keeble, D. J., 609 Kevan, L., 467 Kirby, C., 355 Kiricsi, I., 491 Kiss, I., 367 Kiwi, J., 1703 Klinszporn, L., 155 1 Klissurski, D., 37 Kobayashi, H., 1517 Kobayashi, M., 281 Koda, S., 1267 Kondo, J., 511 Kondo, Y., 11 1 Konishi, Y., 281 Kordulis, C., 1593 Kornhauser, I., 785, 801 Krausz, E., 827 Kristyan, S., 917 Kubelkova, L., 1447 Kubokawa, Y., 751 Kumamaru, T., 1679 Kurimura, Y., 841, 1025 Kuroda, H., 1329 Kusabayashi, S., 11 1 Kuwabata, S., 1587 Lahy, N., 1475 tajtar, L., 19 Lambi, J. N., 1 Laubry, P., 969 Laval, J-M., 941 Lawrence, K. G., 175 Lea, J. S., 1181 Leaist, D. G., 581 Lefever, R., 1013 Lefferts, L., 1491 Lengyel, I., 229 Lewis, T. J., 1531 Leyendekkers, J. V., 397, 1653 Leyte, J. C., 293 Lincoln, S. F., 365 Lindner, Th., 631 Lips, A., 1223 Llewellyn, J.P., 1531 Logan, S. R., 1259 Lycourghiotis, A., 1593 MacKay, R. L., 1145, 1737 Maezawa, A., 851 Malanga, C., 97 Marcus, Y., 175, 1465 Markovid, M., 1301 Maroto, A. J. G., 9 Maruya, K., 511 Mason, D., 473, 483 AUTHOR INDEX Matsumoto, T., 1375 Matsumura, Y., 87 Matsuoka, K., 1277 Mayagoitia, V., 785, 801 McAleer, J. F., 441 McMurray, N., 379 Mead, J., 675 Medda, K., 1501 Mensch, C. T. J., 65 Merkin, J. H., 993 Mills, A., 379, 1691 Mintchev, L., 1423 Mirti, P., 29 Mitsushima, I., 851 Miyakawa, K., 1517 Mohamed, M. A-A., 57, 729, Moiroux, J., 941 Morris, J. J., 865 Morterra, C., 1617 Morton, J. R., 413 Moseley, P. T., 441 Mousset, G., 969 Muhler, M., 631 Murray, B. S., 871 Nagao, M., 1277 Nakamura, T., 1287 Nakamura, Y., 111 Nakao, N., 665 Nakayama, N., 665 Narayanan, S., 521 Nazhat, N.B., 501 Newman, K. E., 1387, 1393 Nicolis, G., 1013 Nishihara, C., 433 Nishikawa, S., 665 Nishio, E., 1639 Nomura, H., 151, 1267 Norris, J. 0. W., 441 Noszticzius, Z., 575 Nucci, L., 97 Ohshima, K., 1639 Ohtani, S., 187 Okabayashi, H., 1639 Okamoto, Y., 851 Okubo, T., 703, 1163, 1171 Oliver, S. W., 1475 Olofsson, G., 551 Ommen, J. G. van, 1491 Onishi, T., 51 1 Ono, Y., 1091 Oosawa, Y., 197 Page, F. M., 1145 Painter, D., 773 Pal, M., 1501 Pan, C.-f., 1341 Pappin, A. J., 1575 Parrott, D., 1131 Passelaigue, E., 17 1 3 Patterson, D., 1603 Pelizzetti, E., 261 Penar, J., 739 Pezzatini, G., 367 Piccini, S., 331 1349 Pichat, P., 261 Pickl, W., 1311 Piekarski, H., 529, 591 Pilarczyk, M., 1551 Pilbrow, J. R., 1475 Pointud, Y., 959, 1713 Pota, G., 215 Preston, K.F., 413 Prior, D. V., 865 Quist, P-O., 1033 Radulovic, S., 1243 Rajam, S., 1349 Rajaram, R. R., 391 Renuncio, J. A. R., 539 Rhodes, C. J., 1187 Riva, A., 1423 Rochester, C. H., 309 Rojas, F., 785, 801, 1455 Ross, J. R. H., 1491 Rowlands, C. C., 1723 Rubio, R. G., 539 Saadalla-Nazhat, R. A., 501 Saito, M., 1025 Saito, Y., 275 Sakamoto, Y., 459 Sakata, Y., 511 Salvagno, S., 1531 Sato, T., 275 Sauer, H., 617 Sawabe, K., 321 Sayari, A., 413 Sbriziolo, C., 207 Schelly, Z. A., 575 Schiffrin, D. J., 561 Schiller, R. L., 365 Schlenoff, J. B., 1123 Schlogl, R., 631 Schmelzer, N., 931 Schulz, R. A., 865 Schwarz, W., 1703 Scott, S. K., 993 Seidl, V., 1447 Sellers, R. M., 355 Senna, M., 1199 Senoda, Y., 1091 Sermon, P.A., 391 Serpone, N., 261 Shindo, H., 433 Sidahmed, I. M., 1153 Sinclair, G. R., 1475 Singh, B., 1729 s’Jacob, K. J., 1509 Smith, E. R., 899 Smith, T. D., 1475 Sokolowski, S., 19, 739 Somsen, G., 529 Soriyan, 0. O., 1 Stainsby, G., 871 Stevens, J. C. H., 165 Stirling, C. J. M., 1531 Stone, W. E. E., 117 Stramel, R. D., 1287 Subba Rao, M., 1703 Sykes, A. F., 1575AUTHOR INDEX Symons, M. C. R., 609, 1181, Szamosi, J., 917 Taga, K., 1639 Tagawa, T., 923 Takada, T., 765 Takagi, Y., 1025 Takato, K., 841 Tanaka, F., 1083 Tanaka, K., 601 Tanaka, K-i., 601 Tardajos, G., 1603 Taylor, D. M., 1531 Taylor, P. J., 865 Tewari, J., 1729 Thampi, K. R., 1703 Theocharis, C. R., 1509 Thomas, J. K., 1287 Thomas, J. M., 617, 631 Tissier, C., 95 1, 969 Tofield, B. C., 441 Torres-Sanchez, R-M., 117 1187 Townsend, R.P., 687 Tra, H. V., 1603 Trifiro, F., 237, 1405, 1423 Tsuchiya, S., 765 Tsukamoto, K., 1639 Twiselton, D. R., 1145 Uematsu, R., 111 Uma, K., 521 Unwin, P. R., 473, 483 Vaccari, A., 1405, 1423 van Rensburg, L. J., 131 I van Veen, J. A. R., 65 van Wingerden, R., 65 Varani, G., 979 Vasaros, L., 367 Vazquez-Gonzalez, M. I., 647 Vidoczy, T., 1075 Viguria, E. C., 255 Vink, H., 133 Viswanathan, B., 365 Vogel, V., 1531 Vordonis, L., 1593 Walker, R. A. C., 255 Ward, J., 713 Wells, C. F., 815, 1153 Welsh, M. R., 1259 Williams, B. G., 617, 631 Williams, D. E., 441 Williams, R. A., 713 Winstanley, D., 1741 Wood, N. D., 1 1 13 Wormald, C. J., 1437 Wyn-Jones, E., 773 Yamada, Y., 751 Yamasaki, S., 1679 Yamashita, S., 1083 Yao, S., 1375 Yoneyama, H., 1587 Yoshida, S., 87 Zecchina, A., 751 Zeitler, E., 617, 631 Zelano, V., 29 Zielinski, R., 151 Zundel, G., 885THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY GENERAL DISCUSSION No.86 Spectroscopy at Low Temperatures University of Exeter, 13-15 September 1988 Organising Committee: Professor A. C. Legon (Chairman) Dr P. B. Davies Dr B. J. Howard Dr P. R. R. Langridge-Smith Dr R. N. Perutz Dr M. Poliakoff The Discussion will focus on recent developments in spectroscopy of transient species (ions, radicals, clusters and complexes) in matrices or free jet expansions. The aim of the meeting is to bring together scientists interested i n similar problems but viewed from the perspective of different environments. The Introductory Lecture will be given by G.C. Pimentel and speakers include: L. Andrews, K. H. Bowen, B. J. Howard, L. B. Knight Jr, E. Knozinger, D. H. Levy, J. P. Maier, J. Michl, M. Moskovits, A. J. Stace, M. Takami, J. J. Turner, M. Poliakoff, A. J. Barnes, J. M. Hollas, M. C. R. Symons and P. Suppan. The final programme and application form may be obtained from: Mrs Y. A. Fish, The Royal Society of Chemistry, Burlington House, London W1V OBNTHE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY WITH THE ASSOCIAZIONE ITALIANA DI CHIMICA FISICA, DIVISION DE CHlMlE PHYSIQUE OFTHE SOCIETC FRANCAISE DE CHIMIE AND DEUTSCHE BUNSEN GESELLSCHAFT FUR PHYSIKALISCHE CHEMIE JOINT MEETING Structure and Reactivity of Surfaces Centro Congressi, Trieste, Italy, 13-16 September 1988 Organising Committee: M.Che V. Ponec F. S. Stone G. Ertl R. Rosei A. Zecchina The conference will cover surface reactivity and characterization by physical methods: (i) Metals (both in single crystal and dispersed form) (ii) Insulators and semiconductors (oxides, sulphides, halides, both in single crystal and dispersed forms) (iii) Mixed systems (with special emphasis on metal-support interaction) The meeting aims to stimulate the comparison between the surface properties of dispersed and supported solids and the properties of single crystals, as well as the comparison and the joint use of chemical and physical methods. Further information may be obtained from: Professor C. Morterra, lnstituto di Chimica Fisica, Corso Massimo D'Azeglio 48, 10125 Torino, Italy. THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY SYMPOSIUM Orientation and Polarization Effects in Reactive Col I isions To be held at the Physikzentrum, Bad Honnef, West Germany, 12-14 December 1988 Organising Committee: Dr S.Stoke Professor J. P. Simons Dr K. Burnett Dr H. Loesch The Symposium will focus on the study of vector properties in reaction dynamics and photodissociation rather than the more traditional scalar quantities such as energy disposal, integral cross-sections and branching ratios. Experimental and theoretical advances have now reached the stage where studies of Dynamical Stereochemistry can begin to map the anisotropy of chemical interactions. The Symposium will provide an impetus to the development of 3-D theories of reaction dynamics and assess the quality and scope of the experiments that are providing this impetus. Contributions for consideration by the Organising Committee are invited in the following areas: (A) Collisions of oriented or rotationally aligned molecular reagents (B) Collisions of orbitally aligned atomic reagents (C) Photoinitiated 'collisions' in van der Waals complexes (D) Polarisation of the products of full and half-collisional processes The preliminary programme may be obtained from: Mrs Y.A. Fish, The Royal Society of Chemistry, Burlington House, London W1V OBN. Professor R. N. Dixon Professor R. A. Levine (viii)THE FARADAY DIVISION OF THE ROYAL SOCIETY OF CHEMISTRY GENERAL DISCUSSION No. 87 Catalysis by Well Characterised Materials - University of Liverpool, 11-13 April 1989 Organising Committee: Professor R.W. Joyner (Chairman) Professor A. K. Cheetham Professor F. S. Stone The understanding of heterogeneous catalysis is an important academic activity, which compliments industry's continuing search for novel and more efficient catalytic processes. The emergence of relevant, in particular in situ techniques and new developments of well established experimental approaches to catalyst characterisation are making a very significant impact on our knowledge of catalyst composition, structure, morphology and their inter-relationships. Well characterised catalysts, which will be the subject of the Faraday Discussion, include single-crystal surfaces, whether of metals, oxides or sulphides; crystalline microporous solids, such as zeolites and clays, and appropriate industrial catalysts.The elucidation of structure/function relationships and catalytic mechanism will be important aspects of the scientific programme. Contributions describing novel methods for synthesising well characterised catalysts and also reporting important advances in characterisation techniques will also be welcome. Contributions for consideration by the Organising Committee are invited and abstracts of about 300 words should be sent by 31 May 1988 to: Professor R. W. Joyner, Leverhulme Centre for Innovative Catalysis, Department of Inorganic, Physical and Industrial Chemistry, University of Liverpool, Grove Street, P. 0. Box 147, Liverpool L69 3BX. Full papers for publication in the Discussion volume will be required by December 1988.Dr. K. C. Waugh Professor P. B. WellsJOURNAL OF CHEMICAL RESEARCH Papers dealing with physical chemistrylchemical physics which appear currently inJ. Chem. Research, The Royal Society of Chemistry's synopsis + microform journal, include the following: An E.s.r. Study of the Radiolysis of Acetylenic Acids and Esters in a Freon Matrix J. Rhodes and Martyn C. R. Symons (1 988, Issue 1 ) Imbibition of Sodium Nitrate by Zeolite Na-Y at 25 "C and Christopher Kevin R. Franklin, Barrie M. Lowe Gordon H. Walters (1 988, Issue 1 ) The Solubility of Carbon Dioxide in Mixtures of Water and Acetone Robert W. Cargill, Donald E. MacPhee and Kenneth Patrick (1988, Issue 1) Correlation Analysis of the Reactivity in the Oxidation of Aromatic Aldehydes by N-Bromoacetamide Anita Gupta, Sandhya Mathur and Kalyan K.Banerji (1 988, Issue 1 ) An E.s.r. Study of Azoalkane Radical Cations Christopher J. Rhodes and Pieter W. F. Louwrier (1988, Issue 1 ) Electrochemical Studies of some Nickel(I1) Complexes of the Type [Ni(NNS)(Heterocycle)-] and [Ni2(NNS)~-(Heterocycle)-][C104] (1 988, Issue 1 ) Sanat K. Mandal, Parimal Paul and Kamalaksha Nag Influence of the Acid-strength Distribution of the Zeolite Catalyst on the t-Butylation of Phenol Avelino Corma, Hermenegildo Garcia and Jaime Primo (1 988, Issue 1 ) The Effect of Nitric Oxide on the Kinetics of Decomposition of Thionitrites Garley and Geoffrey Stedman (1988, Issue 2) Michael S. Kinetics of the Solvolysis of Chlorapenta-aminecobalt(lll) Ions in Water and in Water - Propan-2-01 Mixtures Kamal H.Halawani and Cecil F. Wells (1 988, Issue 2) Evaluation of Broyden - Fletcher - Goldfarb - Shanno (BFGS) Variable Metric Method in Geometry Optimisation using Semi-empirical SCF-MO Procedures Dimitris K. Agrafiotis and Henry S. Rzepa (1 988, Issue 3)FARADAY DIVISION INFORMAL AND GROUP MEETINGS Electrochemistry Group with The Society of Chemical Industry Electrolytic Bubbles To be held at Imperial College, London on 31 May 1988 Further information from Professor W. J. Albery, Department of Chemistry, Imperial College of Science and Technology, South Kensington, London SW7 2AZ Electrochemistry Group with The Society of Chemical Industry Chlorine Symposium To be held at the Tara Hotel, London on 1-3 June 1988 Further information from Dr S. P. Tyfield, Central Electricity Generating Board, Berkeley Nuclear Laboratories, Berkeley, Gloucestershire GL13 9BP Neutron Scattering Group Postgraduate Informal Neutron Conference To be held at the University of Keele on 11-13 July 1988 Further information from Professor C.R. A. Catlow, Department of Chemistry, University of Keele, Keele, Staffs ST5 5BG Colloid and Interface Science Group with the Biochemical Society Dynamic Properties of Biomolecular Assemblies To be held at the University of Nottingham on 20-22 July 1988 Further information from Dr S. E. Harding, School of Agriculture, Unversity of Nottingham, Department of Applied Biochemistry, Sutton Bonington LE12 5RD Gas Kinetics Group Xth International Symposium on Gas Kinetics To be held at University College, Swansea on 24-29 July 1988 Further information from Dr G.Hancock, Physical Chemistry Laboratory, South Parks Road, Oxford OX1 3QZ Electrochemistry Group with the Electroanalytical Group and the Society of Chemical Industry Electrochemcial Dynamics To be held at the University of Strathclyde on 5-10 September 1988 Further information from Dr S. P. Tyfield, CEGB, Berkeley Nuclear Laboratories, Berkeley, Gloucestershire GL13 9PB Statistical Mechanics and Thermodynamics Group Dense Fluids To be held at the University of Cambridge on 14-16 September 1988 Further information from Dr P. Francis, Department of Chemistry, University of Hull, Hull HU6 7RX Carbon Group with the Carbon and Graphite Group of the SCI Carbon 88 To be held at the University of Newcastle upon Tyne on 18-23 September 1988 Further information from The Conference Secretariat, Carbon 88, Society of Chemical Industry, 14/15 Belgrave Square, London SWlX 8PS Division Autumn Meeting: Polymerisation and Polymer Behaviour To be held at the University of Birmingham on 20-22 September 1988 Further information from Professor I.W. M. Smith, Department of Chemistry, University of Birmingham, PO Box 363, Birmingham B15 21T Colloid and Interface Science Group Structure in Colloidal Systems and its Characterisation To be held at the university of Bath on 21-23 September 1988 Further information from Dr R. Buscall, ICI plc, Corporate and Colloid Science Group, PO Box 11, The Heath, Runcorn WA7 4QE Polymer Physics Group jointly with Physical Crystallography Group Diffraction from Polymers To be held at the Geological Society, London on 30 November 1988 Further information from Dr M. Richardson, National Physical Laboratorv, Teddinaton, Middlesex lW11 OLWPolar Solids Group with the Applied Solid State Chemistry Group Computer Modelling of Inorganic Solid Structures To be held at the Scientific Societies' Lecture Theatre, London on 2 December 1988 Further information from Dr A. E. Comyns, R & D Department, Laporte Industries Ltd., Moorfield Road, Widnes WA8 OQJ Colloid and Interface Science Group Aggregation in Colloidal Systems To be held at the Scientific Societies' Lecture Theatre, London on 16 December 1988 Further information from Dr R. Buscall, ICI Corporate Colloid Science Group, P.O. Box 11, The Heath, Runcorn, Cheshire WA7 4QE Neutron Scattering Group Muon Spectroscopy To be held at the University of Nottingham on 20-22 December 1988 Further information from Dr S. Cox, Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire OX1 1 OQX (xii)

ISSN:0300-9599    

DOI:10.1039/F198884BP063

出版商:RSC    

年代:1988

数据来源: RSC

摘要:

J . Chern. Soc., Faraday Trans. I, 1988, 84(5), 1287-1300 Photophysical and Photochemical Properties of CdS with Limited Dimensions Rodney D. Stramel, Takashi Nakamura and J. Kerry Thomas* Chemistry Department, University of Notre Dame, Notre Dame, IN 46556, USA Laponite, porous Vycor glass and molecular sieves, faujasite X and sodalite, have been used to prepare small particles of CdS with limited dimensions. Laponite limits Done dimension of the CdS particles by the interplaner spacing of 11.5 A, Porous Vycor glass limits two dimensions of the CdS particles to < 40 A. Molecular sieves were used to limit the particle size to the size of the cavity; however, the particles constructed were larger than the cavity dimensions. The absorption onset and main emission wavelength increase with increasing Cd2+ concentration.The spectral properties of PbS are also reported. Methyl viologen and Cu2+ quench two different emission bands arising from CdS on laponite in solution. Methyl viologen quenches the high-energy emission, thought to arise from exciton recombination, while Cu2+ efficiently quenches a low-energy emission thought to arise from S2- deficiencies. Methyl viologen shows dynamic-type kinetics, while the static- type kinetics observed from Cu2+ gives an estimate of the particle size. The spectroscopic properties of the particles are studied as a function of cadmium concentration and discussed in terms of particle size. During the past three years there has been significant increase in interest in the photophysics and photochemistry of colloidal semiconductors.Much of this work deals with non-aqueous systems or aqueous systems in which the colloidal particles are stabilized by large amounts of surfactant. The electron-hole pairs produced on illumination can be exploited to study electron transfer at the solid-liquid interface. The size of the colloid greatly influences the spectral properties as compared to single crystals or large particles. Typically the absorption spectrum onset shifts from 520 nm (2.4 eV) to higher energy and the spectra become structured. The luminescence arising from these particles is also shifted to higher energy. These unique spectral properties are said to result from quantum-mechanical effects which affect the energy level of the conduction band. Indeed small particles of CdS,lP7 ZnS,8 PbS,g and various other material^^^-'^ have been prepared in either aqueous or non-aqueous systems.The spectral properties have also been explained by quantum-mechanical calculations. 14-17 Several others have indicated that CdS may be prepared conveniently in constrained systems such as cellulose,1s p~lyurethane,'~ Nafion2' and porous Vycor glass.21 It has been shown that the particles located in these systems exhibit unique and enhanced spectral properties. The spectral properties of CdS incorporated into soda-lime silicate glass was studied in 192622 and further investigated in 1946.23 The data reported indicate that the samples contained very small CdS aggregates, confirmed by a blue shift in both the absorption onset and luminescence band.The unique spectral properties observed were explained by polarization of S2- by Cd2+. In this paper we extend this work, i.e. prepare and study the spectral properties of very small particles of CdS and PbS in restricted and constrained environments. This is accomplished by precipitation of CdS and PbS in porous Vycor glass, zeolites or on clay surfaces. The dimensions of the CdS particles are limited by the nature of the support, 12871288 Photophysical and Photochemical Properties of CdS a long single dimension in porous Vycor glass, a two-dimensional thin sheet on synthetic clay and three small dimensions in the cage of the zeolite. Experiment a1 The materials used were as follows: cadmium sulphide (99.99 + YO pure, Aldrich Chemical Co.), cadmium chloride (99.999 YO Aldrich Chemical Co.), cadmium perchlorate (G.Frederick Smith Co.), lead perchlorate (Aldrich Chemical Co.), hydrogen sulphide (Matheson) and copper sulphate (J. T. Baker Chemical Co.), methyl viologen (N,N'-dirnethyL4,4'-bipyridinium dichloride) (Aldrich Chemical Co.), which was recrystallized three times from methanol, laponite (Laporte Industries, England) faujasite X, sodalite (Exxon Research Co.) and porous Vycor glass no. 7930 (Corning Glass Works). Cadmium or Lead Sulphide incorporated in Vycor Glass Incorporation of CdS or PbS into porous Vycor glass was accomplished by soaking pre- cut sections of the prepared glass in a stirred solution of Cd(ClO,), or Pb(CIO,), for 24 h. The sample was then rinsed and vacuum dried. The glass was rapidly immersed in a solution of isopropyl alcohol saturated with H,S.The CdS on the surface of the glass was removed with dilute HC1. The isopropyl alcohol was removed from the glass pores under vacuum. The glass sample was kept in the dark. Cadmium or Lead Sulphide on Laponite Laponite is a synthetic hectorite with a cation exchange capacity of 0.8 mmolg-l for monovalent cations. Laponite is easily dispersed and solutions up to 20 g dm-3 are transparent. Sodium is the exchangeable cation and can be replaced by most other inorganic and organic cations. To a solution of colloidal laponite a solution of Cd(ClO,), or Pb(ClO,), was added. The solution was stirred for several hours and dried using a rotoevaporator. The powder was dried overnight at 110 "C and stored over Linde MS 5A until further use.The sulphides of the cation-exchanged Laponite were prepared by exposing the dry Laponite powder to a stoichiometric amount of H2S in an oxygen-free environment, and allowing them to react in the dark. After several hours (4-24 h) of reaction time the samples were purged with nitrogen and studied. Cadmium or Lead incorporated in a Zeolite Matrix Preparation I Typically 5 g of Na faujasite X was mixed with 5 g Cd(NO,), in 100 cm3 aqueous ammonia. The sample was filtered and washed with aqueous ammonia, dried at 100 "C for 1 h to remove the water and at 300 "C for 1 h to remove the NH,. This procedure was repeated. The sample is 90 % exchanged with Cd'' in sites I, 1', I1 and I11 [fig. 2(A), see later]. Preparation 2 Typically 5 g of Na faujasite X in 100 cm3 water was adjusted to pH 7.0 with HCl.To the solution 5 g of Cd(NO,), or Pb(NO,), was added and stirred for 30 min. The sample was filtered and dried at 100 "C for 1 h then at 300 "C for 1 h. This procedure was repeated. The exchanged cations are the same for preparation 1.R. D. Stramel, T. Nakamura and J. K. Thomas 1289 Cadmium incorporated into Sodalite The exchange of cations in sodalite is carried out using a molten salt. Typically 5 g of sodalite was mixed with 5 g Cd(NO,),, placed in a Teflon bomb and heated at 100 "C overnight. After cooling, the slurry was washed with water, filtered and dried at 100 "C for 1 h. The sulphides of the cation-exchanged zeolites were prepared by exposing the dry sample to a stoichiometric amount of H,S in an oxygen-free environment. After a reaction time of several hours (sometimes at elevated temperatures) the samples were purged with nitrogen and studied immediately.Instrumentation A Lambda-Physik N, laser with pulse duration 8 ns, energy 6 mJ and wavelength 337.1 nm (main line) was used to study the emission decay. The short-lived transients produced were monitored by fast spectrophotometry (response < 1 ns) and the data captured by a Tektronix 7912A digitizer with subsequent processing by a 4052 minicomputer. Steady-state absorption and emission spectra were recorded on a Perkin- Elmer 552 spectrophotometer and a Perkin-Elmer MPF44B spectrofluorimeter, interfaced to a 405 1 minicomputer through an analogue- to-digital converter.X-Ray diffraction data were obtained with a Diano X-ray diffractometer using Cu Kcc radiation. Results and Discussion CdS on Laponite The reflectance absorption spectra of CdS-laponite powder is shown in fig. 1(A). At high concentration, 0.5 mmol CdS (g laponite)-l, an absorption onset at 510 nm is observed, suggesting large particles. As the concentration is decreased, the absorption onset exhibits a blue shift, indicative of smaller particles. The emission spectra of the samples are shown in fig. 1(B). At high concentration, 0.5 mmol CdS (g laponite)-l, the spectrum is broad, spanning 250 nm, possibly owing to a large distribution of particle size. The emission peak at 560-580 nm is thought to arise from e-/h+ pair recombination on the surface of CdS.,' As the exchanged concentration is decreased, the emission spectra exhibit a blue shift and become narrower.These data, at low concentration of CdS, are similar to that reported for small colloidal particles.l-' The X-ray diffraction sgectra of the cadmium-exchanged laponite had a (001) interplaner spacing of 1 1.5 A corresponding to a single layer of water absorbed between the sheets. This is the same spacing obtained in a sample of laponite treated in the same manner as the Cd2+-exchanged samples. This indicates that the absorbed Cd2+ does not effect the interplanar spacing. On exposure of the samples to H,S the interplanar spacing again did not change, indicating thoat one of the dimensions of the CdS particle is restricted by the clay sheets to 11.5 A. However, no diffraction pattern is observed for CdS.This result suggests that either the CdS formed is to small to diffract X-rays or that it is amorphous in nature. The time-resolved luminescent decays are shown in fig. 1 (C). The emission decays do not follow a simple exponential dependence on time. At long wavelengths (low energy) the luminescent lifetimes are long (several hundred nanoseconds), while that at short wavelengths (high energy) the luminescent lifetimes are short (tens of nanoseconds). It is suggested that the luminescence arises from the electron-hole pair recombination on the surface of the particle. Short-wavelength luminescence arise from small particles. The e-/h+ pairs produced on excitation quickly recombined on the surface. Long-1290 Photophysical and Photochemical Properties of CdS 0.4 - (A1 8 .O - n I 0.0' 120 160 200 0 40 80 Time/ns 0.5 I I I c, .- 9 6 - !a E -5 - v A c,- .- c.' I ' 0.0 ' ' ' " a . ' Fig. 1. (A) Reflectance absorption spectra of CdS on laponite (mmol g-') : (a) 0.05, (6) 0.1, (c) 0.2, ( d ) 0.3 and (e) 0.5. (B) Emission spectra of CdS on laponite (mmol g-l) (a) 0.05, (b) 0.1, (c) 0.2, ( d ) 0.3 and (e) 0.5. (C) Plot of In(intensity) us. time for the luminescent decay at different wavelengths (in nm). 6.0 4.0 2.0 0.0 wavelength luminescence arises from large particles. The e-/h+ pairs have a longer distance to diffuse before they recombined giving rise to a long-lived luminescence. A simple calculation to determine the number of CdS-laponite particles shows that the CdS particles must b: small. If we assume that the laponite particle has the dimensions 700 x 700 x 10 A (from electron microscopy), a volume of 4.9 x cm3 per particle is obtained; given that the density of laponite is 1.2 g a value of 2.45 x 1017 particles of laponite per gram of laponite is obtained.It is establishedz4 that the cation exchange capacity of laponite is 0.8 mmol g-l for monovalent cations; these sites are clustered in islands, and the concentration of the islands was determinedz5. 26 as ca. 0.07 mmol (g laponite)-l or 3.25 x lo1' islands (g laponite)-l, which gives 133 islands per particle. Table 1 lists the number of CdS-laponite particles and CdS-islands. Based on this calculation, the CdS particles must contain only a few molecules of CdS and are very small. If it is assumed that the CdS is oply located on the faces of the clay sheets and not on the edges a total area of 9.8 x lo5 A' per laponite particole is calculated, from which the average distance between the islands is calculated as 86 A.This implies that there is little interaction between islands of CdS.R. D. Stramel, T. Nakamura and J. K . Thomas Table 1. [CdS]/laponite particle ratio CdS CdS /mmol (g laponite)-' /laponite particle CdS/island 0.0 I 0.05 0.1 0.2 0.3 0.5 24 122 245 490 735 1224 0.18 0.92 1.85 3.68 5.52 9.2 1291 Fig. 2. (A) Absorption spectra of CdS on laponite colloid: (b) 5 g dm-3 of 0.1 mmol g-', (c) 2.5 g dmW3 of 0.2 mmol g-l, ( d ) 1.25 g dmP3 of 0.3 mmol g-' and (e) 1 g dm-3 of 0.3 mmol g'. (B) Emission spectra of CdS (mmol g-l) on laponite (5 g dm-3) colloid: (a) 0.05, (b) 0.1, (c) 0.2 and ( d ) 0.3.1292 Photophysical and Photochemical Properties of CdS 400 4 80 560 640 720 800 wavelength/nm 10.0) 1 , I I 40 0 4 80 560 640 720 800 wavelength/nm Fig.3. Change in emission spectra with time. (A) 5 gdm-3 colloid of 0.05 mmol CdS (g laponite)-' at time (a) 5 min, (b) 7 h, ( c ) 27 h and ( d ) 72 h after preparation of the colloid. (B) 5 g dmP3 colloid of 0.3 mmol CdS (g 1aponite)-' at time (a) 5 min, (b) 6 h, (c) 19 h and ( d ) 72 h after preparation of the colloid. CdS on Colloidal Laponite The solid samples of CdS on laponite were suspended in water, making a transparent colloidal solution that scatters very little light. The absorption spectrum of these solutions are shown in fig. 2(A).For comparison, the concentration of CdS is normalized. The spectra are red-shifted when compared with the spectra of the dry powders. All spectra have absorption onsets well below 520 nm; as the concentration of CdS is decreased the absorption onset blue shifts. The emission spectra of these solutions are shown in fig. 2(B). Water has a significant effect on the emission spectra. All emission peaks observed on the dry powder areR. D. Stramel, T. Nakamura and J. K. Thomas 1293 [MVz'l/ 10" rnol drn-3 Fig. 4. Quenching of the 470 nm emission from a 5 gdm-3 solution of 0.05 mmol CdS(g laponite)-' by MV2+. quenched by water, as expected if the luminescence arises from the surface of CdS, where water can interact with the charge carriers. However, there are two peaks observed at ca.470 and 680 nm. It is well established that the emission at 680 nm is due to sulphur defi~iencies.~~ The peak at 470 nm is thought to arise from exciton recombination from small particles,' and is not affected by water. Luminescence lifetimes were < 6 ns at all wavelengths. The ratio and intensity of these two peaks change with time as shown in fig. 3. No change in absorption spectra was observed. The peak remaining at 470 nm indicates that there are small particles which do not grow in size owing to the protection of the laponite layers. Quenching of CdS Luminescence by the Methyl Viologen Dication, MV2+ Fig. 4 shows the effect of MV2+ on the emission intensity of CdS. The emission at 470 nm is efficiently quenched by MV2+. Since MV2+ is located between the layers28 it can efficiently quench the 470 nm band.The quenching of the emission band at 680 nm is less efficient. The emission at 680nm from a 5 g dm-3 solution of 0.3 mmol CdS(g laponite)-l is only 12 % quenched with 5 x mol dm-3 MV2+, after which flocculation occurs. This quenching behaviour can be explained as follows: The MV2+ is found between the clay layers; owing to the negative charge of the clay, it competes with the photogenerated holes for the electrons, a process which leads to a decrease in luminescence intensity. The emission at 680 nm is inefficiently quenched either because of unfavourable energetics or because the MV2+ is located in a region (between the clay layers) where favourable charge transfer cannot occur. A linear plot of ln(Zo/Z) us.[MV2+] for quenching of the 470 nm emission indicates dynamic-type kinetics, suggesting that the MV2+ remains mobile. Quenching by Cu2' Fig. 5 shows the effect of Cu2+ on the emission intensity. The emission at 680 nm is efficiently quenched by Cu2+, while the emission peak at 470 nm is inefficiently quenched.1294 Photophysical and Photochemical Properties of CdS 1 .a 0 0.5 [Cu2+]/IO" mol dm-3 [c~~+]/lO-~ mol dm-3 Fig. 5. (A) Quenching of the 680 nm emission from a 5 g dm-3 solution of 0, 0.05 mmol CdS (g laponite)-' and A, 0.3 mmol CdS (g laponite)-l by Cu2+. (B) Poisson plot of the quenching data : 0, slope = 5.6 x lo5 dm3 m o t ' and A, slope = 4.34 x lo5 dm3 mol-l. In solution Cu2+ first comes in contact with a face of a clay particle where it is strongly attracted to the CdS particle, forming an insoluble sulphide (insolubility product Ksp = on which it is strongly absorbed.Here it serves as a hole trapping site that ultimately leads to a decrease in luminescence intensity. The emission at 470 nm is inefficiently quenched by Cu2+, indicating that Cu2+ cannot quench the exciton emission. A linear plot of ln(Io/I) vs. [Cu"] for quenching of the 680 nm emission indicates static-type kinetics. From the Poisson kinetic equation In (Z,,/I) = [Q]/[S] a site concentration can be calculated. For a 5 g dmP3 solution of 0.05 mmol CdS (g laponite)-l, [S] = 1.78 x lop6 mol dm-3, and for 0.3 mmol CdS (g laponite), [S] = 2.3 x lop6 mol dmP3, these correspond to 140 oand 652 CdS site-', respectively. Assumipg the volume of molecular CdS is 50 A3, total volumes of 7000 and 32600 A3 can be calsulated for 0.05 and 0.3 mmol (g lapon$e)-', respectively. This gives radii of 15 and 32 A for a disc with height of 10 or 11.9 A and 19.8 A for a sphere.Comparing these calculated radii with those published for the expected absorption onset of small colloidal particles,l6. l7 the values correspond quite to with the absorption onset of dry powders. It is suggested that the red shift in the absorption spectrum upon suspending the samples in water is due to a decrease in energy on the surface owing to the interaction with water. This is not observed in colloidal samples because either they are prepared in organic solvents or the surface is coated with stabilizers in aqueous solution.CdS incorporated in Porous Vycor Glass The absorption spectra of CdS incorporated in porous Vycor glass are shown in fig. 6(A). The spectra are dependent on the concentrationoof CdS. Because of the nature of porous Vycor glass, two dimensions are limited to 40 A, while the third long dimension is dependent on the CdS concentration. At high CdS concentration (0.01 mol dm-3 CdS) the absorption spectrum shows a very sharp onset at 520 nm (2.4 eV), which correlates well with the spectrum exhibited by large crystals of CdS as well as the spectrumR. D. Stramel, T. Nakamura and J . K. Thomas 1295 300 350 400 450 500 550 600 w avelength/nm 100.0 80.0 n c c1 .r( 60.0 .!a W x Y .- 40.0 Y .5 20.0 0 .o 400 500 600 70 0 800 w avelength/nm Fig. 6. (A) Absorption spectra of CdS in porous Vycor glass: (a) (b) (c) and ( d ) mol dm-3.(B) Emission spectra of CdS in porous Vycor glass (a) (b) (c) and ( d ) mol dm-3. previously published for 5 x mol dm-3 CdS in Vycor glass.21 Fig. 6 also shows the effect of decreasing the concentration of CdS in Vycor glass. As the concentration of CdS is decreased, the absorption onset shifts toward the blue and attains an absorption tail. Fig. 6(B) shows the emission spectra of CdS in porous Vycor glass. Oxygen does not effect either the shape or the intensity of the spectra. At mol dm-3 CdS the emission spectra shows two peaks located at 480 and 580 nm. As the concentration is increased, both peaks increase in intensity and the 480 nm peak exhibits a red shift. The higher- energy peak is associated with direct e-/h+ pair recombination from the exciton bands.The 580 nm peak is associated with e-/h+ pair recombination on the surface. This emission band is not observed in colloidal systems owing to the interaction of the charge carriers with the aqueous phase. On addition of water to the porous Vycor glass the emission band at 580nm is quenched and a new band appears at 680nm which is attributed to a sulphide deficiency emission. The lifetime of the emission is short (< 6 ns) in all samples.1296 ( A ) Photophysical and Photochemical Properties of’ CdS 10.0 8.0 h m +I .I E: 6.0- rd I 1 1 I (8) - - - Fig. 7. (A) Absorption spectra of CdS on (a) sodalite, (b) faujasite X (preparation 2) and ( c ) faujasite X (preparation 1). (B) Emission spectra of CdS on (a) sodalite, (b) faujasite X (preparation 2) and (c) faujasite X (preparation 1). CdS on Zeolite Powder To investigate further the photophysical properties of CdS in constrained systems, CdS was prepared in zeolites, specifically faujasite X and sodalite.Sodalite is composed of SiO, and A10, tetrahedra linked by shared corners. Faujasite is composed of sodalite cages joined tetrahedrally through four of the eight hexagonal faces to give hexagonal prisms. Synthetic sodium faujasite with %/A1 ratios between 1 .O and 1.5 and between 1.5 and 3.0 are called X and Y zeolites, respectively. The faujasite X in this study had Si/Al = 1.19. The sodium cations in these zeolites are exchangeable and can be replaced with almost any other inorganic or organic cation as size permits.The diameter of the pores is 0.66 nm in sodalite, 0.66 nm in faujasite X I (b-cages) and 1.3 nm in faujasite X I1 (supercages); thus small particles may be grown inside the cages. The sodium cations were 90% exchanged with cadmium cations.R. D. Stramel, T. Nakamura and J. K. Thomas 1297 Table 2. Temperature dependence on the onset of absorption onset of CdS particles sample onset 298 K onset 77 K A/eV CdS/ laponi tea CdS/porous Vycor glass 5 x mol dm-3b 540 nm (2.29 eV) 510 nm (2.43 eV) -0.14 mol dm-3a 520 nm (2.38 eV) 495 nm (2.51 eV) -0.13 mol dmP3 a 505 nm (2.46 eV) 492 nm (2.52 eV) -0.08 no temperature dependence 1 0-4 mol dm-3 a no temperature dependence CdS/zeolites" no temperature dependence Aldrich CdS" 525 nm (2.36 eV) 500 nm (2.48 eV) -0.12 CdS colloidd no temperature dependence CdS/nafion" 512 nm (2.42 eV) 495 nm (2.5 eV) - 0.08 a This work.Ref. (21). ' Ref. (20). Ref. (33). The absorption spectra of CdS in faujasite X and sodalite are shown in fig. 7(A). Again the spectral properties are different from those of bulk CdS and colloidal CdS. The CdS particles are confined to the cages and thus limited in size. Preparation of small metallic particles of Pt, Pd and Ni29 indicate that the particles size are less than 4 nm in diameter and the zeolite cage remains intact. No X-ray diffraction pattern was observed which could arise from CdS, indicating either small particles or amorphous particles. No change in the zeolite diffraction pattern was observed before or after formation of CdS. The emission spectra are shown in fig. 7(B).A single broad band is observed in both systems. This suggests that the emission arising from the CdS in the 0.66 nm diameter cages in sodalite is the same as that arising from the 1.3 nm diameter cages and 0.66 nm diameter cages in faujasite. The lifetimes of the luminescence of all samples at all wavelengths were < 6 ns. Data reported on the size of Pt particles in faujasite X indicate a decrease in particle size with increasing ion exchange. In all samples a single particle size of narrow distribution was obtained. Based on this information and the spectral data obtained in this study, it is suggested that the particles size of CdS $re similar in both faujasite and sodalite and are probably on the order of 4.0 nm (40 A).Nature of the Systems The three systems discussed in this paper limit the dimensions of the CdS particle. Laponite limits one dimension of theoCdS particle through the interplanar spacing of the clay sheets to not more than 11.5 A, while the other two dimensions depend on the concentration of the CdS. Although it is not possible to determine the size of the CdS particles on the dry powder in these studies, the size of the particle may be obtained in solution through kinetic oanalysis. The sizes obtained for 0.05 and 0.3 mmol CdS (g laponite)-l, 15 and 32 A, respectively, correlate well with the predicted absorption onsets of small colloidal particles when compared to the spectra of dry powders, indicating that most of the particles are protected by the laponite layers.As the temperature is lowered to 77 K the expected shift in the main absorption band ( - 5.0 x loP4 eV K-l) is not observed, indicating that the CdS particles do not possess the photophysical properties of large CdS crystals. It is well d o c ~ m e n t e d ~ ~ ~ ~ ~ that the pobes in porous Vycor glass are honeycombed with tubes which have a diameter of 40 A ; therefore CdS prepared in this system has1298 Photophysical and Photochemical Properties of CdS 220 320 420 520 620 720 220 320 420 520 620 720 wavelengt h/nm 220 320 420 520 620 720 wavelength/nm Fig. 8. (A) Reflectance absorption spectra of PbS on laponite (mmol g-'): (a) 0.01, (b) 0.05, (c) 0.01 and (d) 0.03. (B) Absorption spectra of PbS on Laponite colloid (6) 5 g dm-3 of 0.05 mmol PbS g-l, (c) 2.5 g dmP3 of 0.1 mmol PbS g-l, and (d) 0.83 g dm-3 of 0.83 mmol PbS g-l.(C) Absorption spectra of PbS in porous Vycor glass (a) (b) (c) loP3 and (d) mol dm-3. two dimensions limited to < 40 A and a third dimension dependent on the concentration qf CdS. In fact that amounts of CdS used in this study limit the two dimensions to 4 40 A. The long dimension along the tube might be of some significant length; once again, in these studies it is not possible to determine them. As the temperature is lowered to 77 K, the absorption spectrum shifts to the blue in sample of mol dm-3; however for lo-* mol dm-3 no shift is observed. These data, along yith the data of CdS on laponite, suggest that a minimum of one long dimension > 64 A is needed to obtain the optical properties of large semiconducting crystals.Poreparation of CdS in zeolite supports should have limited the particle growth to 13 A in diameter. oHowever, data on small metal particles indicate that particle size can be as large as 40 A without destroying the zeolite cages. As the temperature is lowered to 77 K no shift in the absorption band is observed, indicating that the particles do not possess the photophysical properties of large CdS crystals. Once again it was not possible to determine the particles size in this support. Table 2 lists the photophysical properties of the CdS particle from this and other studies. andR. D. Stramel, T. Nakamura and J . K. Thomas 1299 Lead Sulphide in Constrained Media The reflectance absorption spectra of PbS prepared on laponite are shown in fig.8(A). All spectra have absorption onsets well below that expected for large PbS crystals (0.37 eV).32 The powders are orange in colour. As the concentration is decreased, the absorption onset shows very little blue shift; however, discrete absorption bands begin to appear in the absorption spectra. No luminescence was detected by our instruments on excitation of the samples. Others have reported shifts in the absorption spectra onset from PbS prepared in acetonitrile ;13 however, no discrete absorption bands are visible in the published spectra. Luminescence has also been reported from this sample. It is suggested that laponite protects the small particles of CdS from further growth. Further evidence came from PbS laponite colloidal suspensions.The solid samples of PbS on laponite were suspended in water, and once again, transparent colloidal solutions were obtained. The absorption spectra of these solutions, with [PbS] normalized, are shown in fig. 8 (B). As with the CdS-laponite, the spectra are red-shifted when compared with the spectra of the dry powders. At high concentrations these spectra have tails that extend beyond 750 nm (the limit of our instrument); however, the absorption bands remain present in the spectra, indicative of small PbS particles. No change in absorption spectra was observed over 72 h. Fig. 9(C) shows the absorption spectra of PbS in porous Vycor glass. At high concentrations the glass is black and transparent. As the concentration of PbS is decreased, the absorption onset shifts to the blue and absorption bands begin to appear in the absorption tail.The spectral data of PbS in these constrained systems are consistent with that of CdS. Conclusions The studies presented in this paper tend to illustrate the usefulness of the various matrices in limiting the dimensions of CdS and PbS. The photophysical properties of the small particles are dependent on the environment. These small particles are different from those prepared in organic solvents, in aqueous solution with large amounts of stabilizer or those prepared in soda-lime glass. The CdS particles are unique in that the dimensions are limited by the nature of the support. The absorption spectral onset, the emission wavelength and the luminescent lifetimes depend on CdS concentration. The data may be discussed in terms of particle size, as indicated in studies performed in soda- lime silica glass several decades ago, and recently confirmed through the use of colloids.We thank Dr D. E. W. Vaughan at Exxon Research Corporation for supplying the molecular sieves and the US. Army Research Office for support of this work via grant no. DAAG 29-83-0129, and also the National Science Foundation. References 1 R. Rossetti, S. Nakahara and L. E. Brus, J . Chem. Phys., 1983, 79, 1086. 2 R. Rossetti, J. L. Ellison, J. M. Gibson and L. E. Brus, J . Chem. Phys., 1984, 80, 4464. 3 R. Rossetti, R. Hull, J. M. Gibson and L. E. Brus, J. Chem. Phys., 1985, 82, 552. 4 A. Fojtik, H. Weller, U. Koch and A. Henglein, Ber. Bunsenges. Phys. Chem., 1984, 88, 969. 5 A.Henglein, Pure Appl. Chem., 1984, 56, 1215. 6 A. Fojtik, H. Weller, Ch-H. Fisher, C. Lume-Pereira, E. Janata and A. Henglein, Ber. Bunsenges. Phys. 7 P . Lianos and J. K. Thomas, Chem. Phys. Lett., 1986, 125, 299. 8 H. Weller, U. Koch, M. Gutierrez and A. Henglein, Ber. Bunsenges. Phys. Chem., 1984, 88, 649. Chem., 1986, 90, 46.1300 Photophysical and Photochemical Properties of CdS 9 A. J. Nozik, F. Williams, M. T. Nenadovic, T. Rajh and 0. 1. Micic, J. Phys. Chem., 1985, 89, 397. 10 A. Fojtik, H. Weller and A. Henglein, Chem. Phys. Lett., 1985, 120, 552. 11 H. Weller, A. Fojtik and A. Henglein, Chem. Phys. Lett., 1985, 117, 485. 12 U. Koch, A. Fojtik, H. Weller and A. Henglein, Chem. Phys. Lett., 1985, 122, 507. 13 J. M. Nedeljkovic, M. T. Nenadovic, 0. I. Micic and A. J. Nozik, J. Phys. Chem., 1986, 90, 12. 14 L. E. Brus, J. Chem. Phys., 1984, 80, 4403. 15 L. E. Brus, J. Chem. Phys., 1983, 79, 5566. 16 H. Weller, H. M. Schmidt, U. Koch, A. Fojtik, S. Baral, A. Henglein, W. Kunath, K. Wiess and 17 L. E. Brus, J. Phys. Chem., 1986, 90, 2555. 18 M. Krishran, J. R. White, M. A. Fox and A. J. Bard, J. Am. Chem. Soc., 1983, 105, 7002. 19 D. Meissner, R. Memming and B. Kastening, Chem. Phys. Lett., 1983, 96, 34. 20 J. P. Kuczynski, B. H. Milosavljevic and J. K. Thomas, J. Phys. Chem., 1984, 88, 980. 21 J. Kuczynski and J. K. Thomas, J. Phys. Chem., 1985, 89, 2720. 22 G. Jaeckel, Z . Tech. Phys., 1926, 7, 301. 23 J. K. Inman, A. M. Mraz and W. A. Weyl, Solid Luminescent Materials (Wiley, New York, 1948), 24 B. S. Neuman and K. G . Sansom, J. Soc. Cosmet. Chem., 1970, 21, 237. 25 T. Nakamura and J. K. Thomas, J. Phys. Chem., 1986, 90, 641. 26 T. Nakamura and J. K. Thomas, Langmuir, 1985, 1, 568. 27 D. Duonghong, J. Ramsden and M. Gratzel, J. Am. Chem. SOC., 1982, 104, 2977. 28 B. K. G. Theng, The Chemistry of Clay-Organic Reactions (Wiley, New York, 1974), p. 148. 29 A. Kleine, P. L. Ryder, N. Jaeger and G. Shultz-Ekloff, J . Chem. Soc., Faraday Trans. 1, 1986, 82, 30 H. E. Thomas, Am. Ceram. SOC. Bull., 1983, 62, 523. 31 W. D. Dozier, J. M. Drake and J. Klafter, Phys. Rev. Lett., 1986, 56, 197. 32 N. B. Hannay Semiconductors (Reinhold, New York, 1959). 33 J. Kuczynski and J. K. Thomas, Lungmuir, 1985, 1, 158. E. Dieman, Chem. Phys. Lett., 1986, 124, 557. p. 182. 205. Paper 6/1686; Received 19th August, 1986

ISSN:0300-9599    

DOI:10.1039/F19888401287

出版商:RSC    

年代:1988

数据来源: RSC

摘要:

J . Chern. Suc., Faraday Trans. I, 1988, 8 4 ( 5 ) , 1301-1310 Precipitation of Calcium Oxalates from High-ionic-strength Solutions Part 6.--Kinetics of Precipitation from Solutions Supersaturated in Calcium Oxalates and Phosphates Milenko Markovid and Helga Furedi-Milhofer * ‘Ruber BoikoviC ’ Institute, Zagreb, Yugoslavia The kinetics of precipitation of calcium oxalate trihydrate (COT) and calcium hydrogenphosphate dihydrate (DCPD) from high-ionic-strength solutions ( I = 0.26 mol dm-3, made up with NaC1) supersaturated in both solid phases [Si(COT) = Pi/Ksn-= 6.86-47.20; S,(DCPD) = 2.10-3.571 have been investigated. Under the experimental conditions employed [e.g. initial total reactant concentrations c(Ca) = c(P0,) = (2.6-2.8) x lo-* mol dmP3, c(C,O,) = (1.5-10) x lop4 mol dm-3; pH, 5; temperature 298 K; magnetic stirring alone or followed by mechanical stirring] COT precipitated first and effectively initiated the precipitation of DCPD.Under the conditions at which the induction periods t,(COT) and t,(DCPD) differed by more than 1 h, previously defined quantitative criteria were employed to determine the influence of excess phosphate and calcium ions on the kinetics of crystal growth and aggregation of COT. It is shown that phosphate ions inhibit crystal growth, but have no influence on aggregation. A comparison with previous results obtained at equimolar total calcium and oxalate concentrations [e.g. c(Ca) = c(C,O,)] shows that in the presence of excess calcium ions the efficiency of the inhibition of crystal growth by aggregation is enhanced, while the collision rate coefficient and rate-controlling mechanism of aggregation are not significantly affected.Precipitation from aqueous solutions supersaturated to different solid phases is a problem arising in many fields of human endeavour, e.g. in technology, environmental and biomedical sciences etc. In biomedical research the deposition of different crystalline phases from biological fluids (e.g. blood serum, saliva, urine) has attracted considerable attention. The system under consideration in this work is relevant to urolithiasis research because urine of patients with disorders of calcium metabolism is often supersaturated in calcium oxalates and phosphates, a state that increases the tendency of these patients to form renal st0nes.l Moreover, of the solid phases that form in this system, calcium hydrogenphosphate dihydrate (CaHPO, .2H,O, DCPD) is a frequent constituent of renal ~ t o n e s , ~ , ~ while the triclinic calcium oxalate trihydrate (CaC20, * 3H,O, COT) is more important as a possible precursor to other crystalline To explain the frequent appearance of several different crystalline phases in renal ~ t o n e s , ~ , ~ ‘ in vitro’ studies on the heterogeneous nucleation6V7 and the kinetics of crystal growth8 of calcium phosphates on calcium oxalate seed crystals and vice versa have been performed.It has been shown that hydroxyapatite [Ca,(PO,),OH] nucleates on COT8 and DCPD on calcium oxalate monohydrate7 seed crystals. The mutual overgrowth of COT and DCPD crystal phases has not yet been investigated. Although of equal importance, the spontaneous precipitation from solutions supersaturated to several crystalline phases has so far received little attention.A comprehensive survey recently completed in our laboratoryg shows the influence of the temperature and the initial concentrations of calcium, oxalate and phosphate ions on the 13011302 Precipitation of Calcium Oxalates composition and morphology of calcium oxalates (the monohydrate and dihydrate) and phosphates [DCPD and octacalcium phosphate, Ca,H,(PO,), - 5H20] precipitated from supersaturated, high ionic strength solutions and observed under conditions ap- proaching equilibrium. In this paper we report the kinetics of spontaneous precipitation of COT and DCPD from solutions supersaturated in both crystalline phases.Attention will be given to their mutual interaction and to the influence of excess calcium and phosphate ions on the kinetics of crystal growth and aggregation of COT. In the interpretation of the kinetic data we use recently developed methods1°-12 for the simultaneous analysis of nucleation induction times, crystal growth and aggregation kinetics in spontaneously precipitating systems. Experiment a1 Materials and Methods Stock solutions of calcium chloride, oxalic acid, phosphoric acid, sodium chloride and sodium hydroxide were prepared by dissolving PA chemicals in triply distilled water and standardized by conventional chemical analy~is.~? lo Samples were prepared by mixing solutions containing known concentrations of the anions (oxalate and/or phosphate ions), preadjusted with sodium hydroxide to pH 5.1-5.2, with equal volumes of calcium chloride solutions equimolar with respect to the phosphate concentration.Sodium chloride was added to all solutions to make up the initial ionic strength after mixing to I = 0.26 mol dm-3. All solutions were thermostatted at 298 K, and those used for particle size analysis were purified by filtering through 0.22 pm Millipore filters. 0.1 K (double-walled vessels) in two parallel sets of experiments for which samples were prepared from the same starting solutions. In the first set of experiments magnetic stirring was employed and the pH was monitored continuously. The appearance of the first precipitate (calcium oxalate) was detected visually by a change in the turbidity of the solutions and was verified by optical microscopy.Subsequent precipitation of calcium phosphate was detected and followed by the change in solution pH.13 In a parallel set of experiments the kinetics of precipitation of calcium oxalate were followed by particle size analysis (Coulter counter Mo TA 2). Solutions were magnetically stirred until the first appearance of the precipitate, subsequently an electrically driven stirring paddle was employed (' mixed ' stirringl4). The composition of the precipitates was ascertained as previouslylO by optical microscopy, t.g.a. and X-ray powder diffraction. The kinetics of precipitation were followed at 298 Treatment of Data The initial supersaturations of the solutions with respect to DCPD and COT were expressed as Si = Pi/Ksp, where Pi is the ionic product and the respective Ksp values are given in table 1 .Ion activities were calculated from the total initial concentrations of all ionic species in solution using ion association and solubility constants listed in table 1 ,15-21 as well as mass balance and electroneutrality equations.22 The calculations were made by successive approximations for the ionic strength and activity coefficients were calculated from the extended form of the Debye-Huckel equation, proposed by D a ~ i e s . ~ ~ From the Coulter counter output data the total volume of precipitated COT, K, was calculated as previously de~cribed.~, The degree of the reaction, a, is defined as VJV,,,, where V,,, = MCOT(cO-cs)/p, M,,, is the molecular mass of COT, p is the density (p = 1.8 g ~ m - ~ ) , ' ~ c, and c, are the calculated initial and equilibrium concentrations of the oxalate species (table 1).The precipitation kinetics of COT were interpreted according to the rate equation :lo (daldt) a-! = KN$ S p (1)M. MarkoviC and H. Fiiredi-Milhofer 1303 Table 1. Thermodynamic ion association constants and solubility products at 298 K equilibria log K ref. H+ + C,Oi- $ HC,O, Na' + C,Oi- s NaC,O, Ca2+ + C,O;-e CaC,O, Ca2+ + CaC,O, + Ca,(C,0,)2+ CaC,O,. 3H,O(s) + Ca2+ + C,Oi- H+ + H,PO, e H,PO, H+ + HPOi- e H,PO, H+ +PO;- g HP0;- Ca2+ + H,PO, e CaH,PO: Ca2+ + HP0i-G CaHPO, Ca2+ + PO:- CaPO, CaHPO, . 2H,O(s) + Ca2+ + HP0;- 4.266 1.035 3.187 3.34" -8.318 2.148 7.199 12.35 1.41 2.74 6.46 - 6.56 15 5 5 16 5 17 18 19 20 20 20 21 a Estimated as being 20 % smaller than the value given in ref.(16) for 310 K, in accordance with the ratio of the values of the constants of the other oxalate complexes determined at 310 and 298 K.5 where Nt is the total number of particles, K is a constant and p is the order of the reaction. The supersaturation, S, has been defined as 1 -a. For dominant crystal growth, eqn (1) reduces to the' crystal-growth equation : (2) Plots of the logarithmic form of eqn (1) may be resolved into parts corresponding to the rate-controlling precipitation processes [fig. 4 (later) and ref. (lo)]. The slope and intercept of the straight-line part (B in fig. 4) give the exponent p and the rate constant K5 in eqn (2). The following quantitative parameters are used for the evaluation of the influence of calcium and phosphate ions.(a) For the rate of crystal growth: the exponent p [eqn (2)]; provided the initial course of the volume (or a) versus time curves is identical, pin > po > ppr (where the subscripts 'in' and 'pr' designate induction and promotion and '0' stands for the control system).12 (b) For the rate and mechanism of aggregation: RZggr, defined as the rate of precipitation at time t,*,,! at which aggregation starts influencing the rate of crystal growth (fig. 4, later); n', which is the slope of the Aa/a* versus time curve; ki, the average collision rate coefficient. Act = a* -a is the effective decrease in a due to aggregation and a* is the corresponding a value calculated for uninhibited crystal growth [in a previous treatment12 we used n = (Aa/a)/At instead]. The crystal growth-rate constant, K, [eqn (2)], is not considered as a quantitative parameter for the following reasons: (i) it is not sensitive enough because large changes in p result in relatively small changes in K,12 and (ii) its value depends on the value and position of the maximum rate (R,,, in fig.4), which cannot be accurately determined by Coulter counter because of the detection limit of the instrument (ca. 1 pm). For calculations of Aa" we use the integrated form of the crystal growth equation26 (daldt) a-f = K,( 1 - a)p. t = KpIp(a) where I&) = l a - i ( l -a)? (2)1304 Precipitation of Calcium Oxalates I,(a) values for p d 4 were deduced from tables published by Nielsen,26 while for p > 4, Ip(a) values were calculated by a computer program for a = 0.02-0.98 (in steps of 0.02).The average collision-rate coefficient, Ej, gives information on the rate-controlling transport mechanism of aggregation and is defined as1' - inN(d,) k . = -~ tlv(d,) (5) where r 2 rzggr, N(d,) is the number of small particles with diameters dd, and N(dJ is the number of all particles with diameters > d,. The division into N(dl) and N(d2) is based on the consideration that in an aggregating system it is possible to define a limiting particle diameter dl such that N(dl) continuously decreases with time, while N(d2) is either invariant or slightly increases, but may be considered constant for the purpose of the ana1ysis.l1 In the following treatment d, has been defined by the above criterion and is therefore slightly different for the phosphate-containing samples and the controls, respectively.Results and Discussion In the following kinetic experiments precipitation was initiated from solutions supersaturated to both calcium phosphates and calcium oxalates (investigated systems, IS). Calcium and phosphate concentrations were equimolar [c(Ca) = c(P0,) = (2.62.8) x mol dmW3] and in large excess over oxalate ions [c(C,O,) = 1.5 x lop4 -1 .O x mol dm-3]. The initial pH was ca. 5, which made DCPD the only precipitating calcium phosphate phase.27 Under the specific stirring conditions employed in both sets of experiments (e.g. magnetic stirring at least until the onset of precipitation) COT was the only precipitating calcium oxalate'o' l4 and it was shown by solid-phase analysis that no significant phase transformation occurred during the kinetic runs.The initial supersaturations with respect to DCPD and COT, respectively, ranged for DCPD S,(DCPD) = 2.62-3.57 and for COT Si(COT) = 6.8647.20. Parallel control experi- ments with only one of the anions in solution (CS, without oxalate and CS, without phosphate ions) have been carried out under the same experimental conditions. The Influence of COT on the Precipitation of DCPD A typical kinetic experiment is shown in fig. 1, curve 1. As expected from a comparison of the initial supersaturations in these experiments [&(COT) = 2.6 Si(DCPD)], COT formed first with an induction period ti = 10-12 min. This precipitate then served as a seed for the precipitation of DCPD, which in the controls (CS,, curve 2) appeared at t = 3 h.The time corresponding to the onset of the pH drop was taken as the induction period characteristic of DCPD precipitation, ti(DCPD). The course of the pH versus time curve gives a qualitative estimate of the rate of precipitation of DCPD. A comparison of curves 1 and 2 shows that ti(DCPD) is significantly shorter in the presence of COT, i.e. the calcium oxalate acts as a specific seed material. This effect was verified in a number of experiments which are listed in table 2. At high initial supersaturations of COT [$(COT) = 26.40 and 47.201 induction periods were t,(COT) % 1 min. Such fast precipitations of COT induced the coprecipitation of some DCPD crystals if S,(DCPD) was 2.6-3.6 (runs 14-16 in table 2; fig.2), i.e. at supersaturations at which DCPD even in the presence of COT seed material was metastable for more than 1 h (runs 5-12 in table 2). Apparently in this experiment the fast nucleation of the less soluble precipitate (COT) has induced co-nucleation of the more soluble phase (DCPD) from metastable solutions of quite low supersaturations. However, since S(DCPD) decreased during this initialM. Markovit and H. Furedi-Milhofer 1305 t/102 s I I I I 111 117 123 129 135 5.1 I ti(DCPD) 1.5 t I I I I I 3 9 60 66 72 78 8L t / l O Z s Fig. 1. pH us. time curves showing the precipitation of DCPD in the presence and absence of COT crystals in solution. Experimental conditions were as given in table 2, runs 6 (curve 1) and 2 (curve 2).ti(COT) and t,(DCPD) are induction periods of the respective crystalline phases. ,-LIP- Table 2. Induction periods of DCPD, t,(DCPD), in the presence and absence of COT precipitate c(C,O,) c(Ca) = c(P0,) t,(COT) Si ti(DCPD) run mol dmd3 /lo-, mol dm-3 pH," /s (DCPD)c /s 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 - 1.5 1.5 1.5 1.5 1.5 1.5 2.0 2.0 6 .O 6.0 6.0 10.0 2.7 2.7 2.6 2.8 2.7 2.7 2.7 2.7 2.7 2.6 2.7 2.7 2.6 2.6 2.6 2.8 - 4.94 4.93 - 5.00 5.00 - 4.93 6.86 4.93 6.86 4.99 6.86 5.00 6.86 5.01 6.86 4.96 6.60 4.95 9.12 4.96 9.12 4.85 26.40 4.96 26.40 5.03 26.40 5.04 47.20 - - 600-720 600-720 600-720 600-720 600-720 600-720 600-720 600-720 0 0 0 0 2.72 11 100 2.66 11 700 2.92 11 820 3.31 9 840 2.66 9 000 2.66 6 600 3.03 4 500 3.1 1 6 300 3.20 8 700 2.66 6 300 2.80 9 900 2.85 8 400 2.10 4 500 2.56 0 3.17 0 3.57 0 I = 0.26 mol dmP3 (made up with NaCI, T = 298 K).a pH established after initial equili- bration. Error & 0.02 units. Si(COT) = [a,(Ca2+) ai(C,Of-)]/K,,(COT). Si(DCPD) = [ai(Ca2+) a,(HPO,"-)]/K,,(DCPD). precipitation, further growth of DCPD crystals stopped, proceeding only after a second 'induction period'. The results were two-step kinetic curves, such as those shown in fig. 2. Both effects (reduction of the induction period of DCPD at low oxalate concentration and coprecipitation with COT at higher oxalate concentration) can be explained if these materials can overgrow epitaxially, i.e. if the lattice misfit between the equilibrium lattice spacings of the substrate and the overgrowing crystalline material is not greater than1306 4.5 Precipitation of Calcium Oxalates - t/ 102 s 6 5L 60 66 72 78 8L I r t I I I I I I 4.7 4.6 - - 10-20 %.2s A comparison of the (010) faces of monoclinic DCPD2’ and triclinic COT30 shows that the misfits between the corresponding parameters (a, c) are 4.7 and 12.7 %.Apparently mutual epitaxial overgrowth of DCPD and COT is possible along the (010) direction. Kinetics of Crystal Growth and Aggregation of COT The large difference in the time of appearance of COT and DCPD in the systems investigated (fig. 1 and table 1) enables one to study the effect of a large excess of phosphate ions on the precipitation kinetics of calcium oxalate which may be followed at ti(COT) < t < t,(DCPD). Moreover, by comparing the respective control systems, CS,, with systems containing equimolar concentrations of calcium and oxalate ions10-12 the influence of excess calcium may also be evaluated.The precipitation kinetics of COT in a system corresponding to system 1 in fig. 1 and the respective control system, CS,, is represented by K and Nt versus time curves (fig. 3) and the corresponding log rate uersus log supersaturation curves (fig. 4). Even a qualitative comparison (fig. 3 ) shows that precipitation is delayed and less abundant in the presence of phosphate ions. Translation of the K versus time curves [curves 1 and 2, fig. 3(a)] to the same starting point shows that their initial course is sufficiently similar that the quantitative criterion : pin > p, > ppr [where p is defined by eqn (2) and section B in fig.41 may be applied [ref. (12), see also the section entitled Treatment of Data]. The corresponding p values : p = 4.8 0.2 for the phosphate-containing systems (four kinetic runs) and p = 3.9k0.1 for the controls (five kinetic runs) are significantly different, showing that crystal growth is inhibited in the presence of phosphate ions. In order to evaluate the effect of excess calcium ions on the crystal growth rate we also compare the results obtained at equimolar reactant concentrations.lo Under these conditions p = 3.6k0.3 in the range of initial reactant concentrations c(Ca) = c(C204) = 5.5 x 10-4-1.6 x mol dm+. The relatively small difference between this value and the value obtained in the excess of calcium ions (p = 3.9 f 0.1, this paper) indicates that calcium ions do not influence the rate of crystal growth of COT.The influence of aggregation on the rate of crystal growth is evident from fig. 4 and 5. It is apparent [section C in fig. 4 and fig. 5(a)] that both in the phosphate-containingM. MarkoviC and H. Fiiredi-Milhofer 1307 t/10* s Fig. 3. Precipitate volume (a) and number of particles (b) of COT as a function of time. Initial total reactant concentrations and temperature: curve 2 as in run 10, table 2, but pH, = 5.00; curve 1, as for curve 2 without phosphate ions. -2.5 Rmax, Rmax, -3.0 - M I v) 1 \ N a n p" s 2 -3.5 Y M - -4 .O -4.5 -0.1 -0.2 -0.3 log ( 1 -a) Fig. 4. Rate us. supersaturation curves calculated from the data shown in fig. 3. 1, without phosphate; 2, in the presence of phosphate ions.(A) nucleation and crystal growth; (B) dominant crystal growth ; (C) crystal growth and aggregation. R,,,, maximum precipitation rate, Rzgggr, rate of precipitation at time at which aggregation starts to influence the rate of crystal growth. Slopes of parts B: p (1) = 3.8, p (2) = 4.8.1308 Precipitation of Calcium Oxalates 0.8 c 0.6 - a! 0.4 0.2 - - a j A a 1 I I I I I 1 I I J 0 6 12 18 24 30 36 L2 L0 t/10* s Fig. 5. Parameters Aa = a*-a (a) and Aa/a* (b) as a function of time. 01 calculated from experimental data, a* calculated for crystal growth uninhibited by aggregation. Conditions of precipitation and connotations as in fig. 3. system and in the control aggregation inhibits crystal growth owing to the large sodium ion concentration in the Fig.4 shows that although in the phosphate- containing system aggregation commenced at higher supersaturation than in the controls, the corresponding precipitation rates were nearly the same, i.e. from all kinetic runs. This apparent discrepancy is a consequence of the difference in the exponents p in the crystal growth equations [eqn (2)] describing sections B of the corresponding rate versus supersaturation curves. This may be understood if one considers that growth inhibition by an additive is usually a consequence of a decrease in the effective surface area by adsorption at the active growth sites31 (the adsorption of phosphate ions at the surface of calcium oxalate crystals has been demonstrated’). The effective surface area in tht phosphate-containing system is then actually lower than that estimated by the term a-5 [eqn (1) and (2) and fig.41. Therefore comparable crystal growth rates in the phosphate-containing system and in the controls have been achieved at different supersaturations (fig. 4). Fig. 5(b) shows that the slopes, n’, of the Aa/a* versus time curves are also equal. From all kinetic runs (four runs for the phosphate-containing systems and five runs for the controls) n’(1S) = n’(CS,) = (1.1 s-’ has been calculated. Thus since Rtggr(IS) = R,*,,,(CS,) (fig. 4) and n’(IS) = n’(CS,) (fig. 5 ) we conclude that the efficiency of aggregation in inhibiting crystal growth is approximately equal in both the phosphate-containing systems and in the controls. From data such as those shown in fig.6 the following values for the collision rate coefficient, Ej [eqn (5)], have been calculated: Ej(IS) = (2.2k0.5) x IO-’s-l and k,(CS,) = (2.5 & 0.6) x lo-’ s-’. The order of magnitude of Ej indicates that under the given experimental conditions the rate-controlling mechanism of aggregation is differential settling. In order to estimate the effect of excess calcium ions on the parameters characterizing aggregation the values for Rzggr, n’ and kj obtained in this are compared with the corresponding values obtained for the equimolar system.’l.12 In doing so one has to take into account that the experimental system involves spontaneous precipitation initiated 0.2) xM. MarkoviC and H. Furedi-Milhofer 1309 5 n N 2r’ ” n B , : =?I 2 1 18 24 30 36 42 t/102 s Fig.6. Data for the calculation of the collision rate coefficient, E, [eqn (5)]. Change in the number of particles : (a) N(d,) and (b) In N(dl) with time. Particle diameters : 1 < dl/pm < 6, d, > 6 p for the system without phosphate (1) and 1 < dJpm < 4.8, d, > 4.8 pm for the system with phosphate ions (2). Conditions of precipitation as in fig. 3. by heterogeneous nucleation upon non-specific impurities, and therefore some scattering of the data is unavoidable. In this and in previous experimental errors have been minimized by running successive parallel experiments from the same stock solutions, but variations in the parameters obtained from different sets of experiments may be appreciably larger. By critical consideration of all available data valuable conclusions may, nevertheless, be drawn.In the following analyses we take into account that the ‘coagulating efficiency’ of an electrolyte (in this case calcium chloride) depends on the concentration of the solid phase,32 and therefore we regard the values obtained in systems of comparable solid- phase concentrations [e.g. c,(Ca) = c,(C204) = 6.0 x and 6.2 x lo-* mol dm-3] as most reliable, but we also consider the values obtained for all other systems. The following results were obtained. (1) The parameter log (Rzggr/s-’) varied between - 3.3 and - 3.9,10-12,33 but for c,(Ca) = ci(C,O,) = 6.0 x lo-* mol dmP3 the value was -3.4 [fig. 1 in ref. (1 1); the ordinate in this figure was erroneously labelled log (Rls-l) instead of log (Rlmin)].(2) The values n’ = (Aa/a*)/At depend on the concentration of the solid phase at tzggr. For c(Ca) = c(C,04) = 6.2 x lo-* mol dm-3 (the most comparable concentration for which a complete set of experimental data is available) n’ = (4.5+ 1) x s-l [recalculated from fig. 4 in ref. (1 l)]. The highest value obtained in any of the equimolar systems was n’ = (7.5 & 1) x lop5 s-l [recalculated from the value for n = (Acc/a)/At reported in ref. (1 2)]. (3) An average collision rate coefficient Ej = (3.9k0.6) x lo-* cm3 s-l fitted all equimolar systems. It is apparent that of the aggregation parameters determined in this investigation only the value n’ = (1 1 s-l differs significantly from the value determined for the corresponding equimolar system. We therefore conclude that in the presence of excess calcium ions (regardless of the presence of phosphate ions) crystal growth is more efficiently inhibited by aggregation than in equimolar systems, while the average 2) x1310 Precipitation of Calcium Oxalates collision rate of the particles and the rate-controlling transport mechanism of aggregation are unaffected. The enhanced inhibition of crystal growth is probably caused by a change in the mode of aggregation (closer packing of the crystals) due to some specific action of the lattice ion, calcium.In general agreement with the above results, previous investigations of the composite system Ca(OH),-H,PO,-H,C,O,-NaCl-H,O under conditions approaching equil- ibriumg have shown that phosphate ions inhibit the precipitation of mixtures of calcium oxalate monohydrate and dihydrate. Apparently the effect of phosphate ions is due to adsorption at the surface of the growing calcium oxalate crystalsg and is not specific with regard to any particular hydrate form.The above investigation shows for the first time that it is possible to obtain quantitative data on the kinetics of growth and aggregation of crystals formed from unseeded solutions supersaturated in two different solid phases. The only condition for such analysis is a large enough difference in induction periods, i.e. in the initial supersaturations of the respective salts. The results obtained are relevant to urolithiasis research because the supersaturations of the test solutions with regard to calcium oxalate and DCPD, their pH and ionic strength are comparable to those found in the urine of potential renal stone formers.' The financial support given by the Self-management Council for Scientific Research of Croatia, Yugoslavia and by the U.S.-Yugoslav Joint Fund for Scientific and Technological Cooperation in collaboration with the National Institutes of Health, Bethesda, MD, U.S.A.(project no. JFP 697) is gratefully acknowledged. References 1 C. Y. C. Pak, Y. Hayashi, B. Finlayson and S. Chu, J. Lab. Clin. Med., 1977, 89, 891. 2 D. J. Sutor and S. E. Wooley, Br. J. Urol., 1972, 44, 532. 3 W. Dosch, Med. Welt, 1978, 29, 39. 4 G. L. Gardner, J. Crystal Growth, 1975, 30, 158. 5 B. Tomaiid and G. H. Nancollas, J. Crystal Growth, 1979, 46, 355. 6 C. Y. C. Pak, Y. Hayashi and L. Arnold, Proc.Soc. Exp. Biol. Med., 1976, 153, 83. 7 J. L. Meyer, Bladder Stone Disease, 1974, 75, 83. 8 P. G. Koutsoukos, M. E. Shehan and G. H. Nancollas, Invest. Urol., 1981, 18, 358. 9 H. Furedi-Milhofer, M. Markovid and M. Uzelac, J. Crystal Growth, 1987, 80, 60. 10 D. Skrtid, M. Markovid and H. Furedi-Milhofer, J. Crystal Growth, 1984, 66, 431. 11 M. Markovid, D. Skrtid and H. Furedi-Milhofer, J. Crystal Growth, 1984, 67, 645. 12 D. Skrtid, M. Markovid and H. Furedi-Milhofer, J. Crystal Growth, 1986, 79, 791. 13 Lj. BreEeviC and H. Furedi-Milhofer, Calc. Tiss. Res., 1972, 10, 82. 14 D. Skrtid, H. Furedi-Milhofer and M. MarkoviC, J. Crystal Growth, 1987, 80, 113. 15 G. D. Picking and R. G. Bates, J. Res. Natl Bur. Stand., 1948, 40, 405. 16 J. R. Burns, B. Finlayson and A. Smith, Invest. Urol., 1980, 18, 165. 17 R. G. Bates, J. Res. Natl Bur. Stand., 1951, 47, 127. 18 F. Ender, W. Teltschik and K. Schafer, 2. Electrochem., 1957, 61, 775. 19 R. M. Smith and A. E. Martell, Critical Stability Constants (Plenum, New York, 1976), vol. 4. 20 A. Chugtai, R. Marshall and G. H. Nancollas, J. Phys. Chem., 1968, 72, 208. 21 E. C. Moreno, T. M. Gregory and W. E. Brown, J. Res. Nut1 Bur. Stand., 1966, 70A, 545. 22 B. Purgarid and Z. Tutek, Anal. Chim. Acta, 1979, 112, 193. 23 W. Stumm and J. J. Morgan, Aquatic Chemistry (Interscience, New York, 2nd edn. 1981). 24 M. MarkoviC and Lj. Komunjer, J. Crystal Growth, 1979, 46, 701. 25 Lj. BreEeviC, D. Skrtid and J. Garside, J. Crystal Growth, 1986, 74, 399. 26 A. E. Nielsen, Kinetics of Precipitation (Pergamon, Oxford, 1954). 27 B. Kosar-Graiid, B. PurgariC and H. Furedi-Milhofer, J. Inorg. NMC~. Chem., 1978, 40, 1877. 28 F. C. Frank and J. M. van der Merwe, Proc. R. Soc. London, Ser. A , 1949, 198, 205. 29 C. A. Beevers, Acta Crystallogr., 1958, 11, 273. 30 S. Deganello, R. A. Kampf and P. B. Moore, Am. Mineral., 1981, 66, 859. 31 S. T. Liu, A. Murwitz and G. H. Nancollas, J. Urol., 1982, 127, 351. 32 E. c. Burton and E. Bishop, J. Phys. Chem., 1920, 24, 701. 33 D. Skrtid, MSc Thesis (University of Zagreb, 1981). Paper 612440; Received 18th December, 1986

ISSN:0300-9599    

DOI:10.1039/F19888401301

出版商:RSC    

年代:1988

数据来源: RSC

摘要:

J . Chern. Soc., Favaday Trans. I, 1988, 84(5), 1311-1328 Hydrocarbon Formation from Methanol and Dimethyl Ether using WO,/Al,O, and H-ZSM-5 Catalysts A Mechanistic Investigation using Model Reagents Graham J. Hutchings,* Lawrence Jansen van Rensburg, Wolfgang Pick1 and Roger Hunter" Department of Chemistry, University of the Witwatersrand, PO Wits, 2050 Johannesburg, South Africa The methanol conversion catalysts H-ZSM-5 and 10 YO WO,/A1,0, show significant differences in their activities as well as product selectivities towards a range of reagents Me-X [X =-OH, -OMe, -1, -OSO,Me, -OP(OMe),O]. WO,/Al,O, is typically two orders of magnitude less active and gives significant yields of methane and hydrogen compared to the zeolite catalyst. Formation of C , , hydrocarbons over WO,/Al,O, with Me,SO, and (MeO),PO provides evidence that the trimethyloxonium ion is an unlikely intermediate in the methanol to hydrocarbon conversion reaction, since these reagents cannot form this type of intermediate.Reactions in the presence of added hydrogen do not show significant differences in the product selectivities, particularly methane, indicating that the methane does not originate from reaction of an intermediate, e.g. methylene carbene, with molecular hydrogen. Significant differences in catalyst behaviour in the presence of added NO and 0, indicate different reaction mechanisms for initial C-C bond formation with both catalysts. The conversion of methanol, or dimethyl ether, into gasoline range hydrocarbons has received considerable attention since 1973 as it represents a process by which liquid hydrocarbon fuels can be formed independently of crude oil, and the process has recently been commercialised in New Zealand.The formation of hydrocarbons from methanol has been known for more than a century, and zinc halides were used as the early catalyst^.^.^ Supported aluminium sulphate4 and phosphorus pentoxide5 have also been cited as catalysts for this reaction. However, the catalyst which has received the most attention in this context is the highly siliceous zeolite of intermediate pore size denoted ZSM-5,6*7 which has been shown to produce premium-grade gasoline from methanol by virtue of its high acidity and shape selectivity. Initially it was Chang and Silvestri' who demonstrated that ZSM-5 converted a wide range of small organic molecules into higher hydrocarbons, and the effect of reaction conditions using ZSM-5 has now been well studied,' producing a considerable amount of literature data.'-'' More recently, Olahl2, l3 and Ip14 have shown that bifunctional acid-base catalysts, such as W0,/A1,03, can also convert methanol into ethene and lower alkenes.Whereas supported tungsten oxide and tungstate catalysts have been extensively investigated for alkene metathe~is'~ and methanol oxidation reactions,l' such catalysts have not been well studied for the methanol conversion reaction. Of particular interest for both W03/A1203 and ZSM-5 catalysts is the mechanism by which the initial carbon-carbon t Present address : Leverhulme Centre for Innovative Catalysis, Department of Inorganic, Physical and Industrial Chemistry, The University of Liverpooll Liverpool L69 3BX.131 11312 Hydrocarbon Conversion using W0,/A120, and H-ZSM-5 bond is formed. To date there has been no detailed comparative mechanistic study of the C , -+ C, transformation step using these two catalysts, whilst it has been previously ~onsidered'~ that the same mechanism operates for this step with both. In this paper we extend our previous mechanistic and report on a comparative study of WO,/Al,O, and ZSM-5 as catalysts. In particular, by using model reagents and probe reactions we demonstrate that the mechanisms of CH, and C-C bond formation are significantly different for the two catalysts. Experimental The acid form of zeolite ZSM-5 (H-ZSM-5) was prepared as previously described.lo Tungsten oxide (10 YO) supported on y-alumina was prepared according to the method of Olah et aZ.13 y-Al,O, (Strem Chemicals, surface area 240 m2 g-') was impregnated with an aqueous solution of ammonium metatungstate using the incipient wetness technique (ammonium paratungstate was found to give inferior catalysts). The catalyst was dried in air at 120 "C for 8 h and calcined at 550 "C for 5-16 h. Catalysts prepared using the longer calcination period gave superior results. Following calcination the catalyst was found to be amorphous by X-ray diffractometry, which is in agreement with previous structure studies of this catalyst. The calcined W0,/A120, was pelleted, without addition of binder, and sieved to give particles (0.5-1.0 mm).10% W03/y~-A120, was also prepared according to the method of Maitra et aZ.22 y-Al,O, was stirred in an excess of aqueous ammonium metatungstate at pH 6.5, 25 "C for 48 h. The catalyst was recovered by filtration, dried at 120 "C for 2 h and calcined at 500 "C for 16 h. H-ZSM- 5 and W03/A1,0, were reacted with methanol and methylating agents using the procedures previously described. 2" Results Reaction of Methanol and Dimethyl Ether over WO,/AI,O, Methanol and dimethyl ether were reacted over WO,/Al,O, using a range of experimental conditions, and the results are given in table 1. For the W0,/A120, catalyst the principal hydrocarbon product under most conditions was methane, as has been previously noted by Olah.'2,13 In addition, hydrogen was formed in comparable yield to that of methane and in general both methane and hydrogen yields increase with increasing reaction temperature and conversion.It is also apparent that methane selectivities were high for methanol as a reagent compared to dimethyl ether at comparable reagent feed rates. These product selectivites are in direct contrast to those observed for methanol conversion using H-ZSM-5 as catalyst when both methane and hydrogen are minor reaction products. Previous studies have shown that methane only becomes a significant product at low conversions'1~20~23 and it is considered to be the primary hydrocarbon product at such conditions. l3 In addition to the differences observed in product selectivities, W0,/A120, and H- ZSM-5 demonstrate considerably different catalytic activities.In general, low reactant feed rates are required to achieve complete conversion for W0,/Al,0,,12-14 and this has been confirmed in this study. With H-ZSM-5 high conversions can be achieved at comparable temperatures with increased reactant flow rates,' and H-ZSM-5 is typically two orders of magnitude more active than WO,/Al,O, for the methanol conversion reaction. In addition H-ZSM-5 exhibits a longer lifetime compared to that of WO,/ A120,. Under the conditions cited in this study the W0,/A120, became rapidly discoloured owing to the laydown of carbonaceous deposits, and a significant irreversible decrease in the catalyst activity was observed from ca. 10 h reaction time with MeOH as reactant. For MeOMe, by virtue of the higher feed rates used, the lifetime was onlyG.J . Hutchings et al. 1313 Table 1. Reaction of MeOH over 10% W03/A1203 hydrocarbon selectivity (YO by mass) conversion H Z b H2/CH4 WHSV" T I T (YO) CH, C,H, C2H6 C3H6 C,H, C, C,+ (mol%) molar ratio 0.008 0.008 0.008 0.008 0.002 0.017 0.30 1.48 14.8 275 0.8 29.2 26.4 300 17.2 22.8 13.2 350 38.4 28.3 13.6 400 97.5 31.7 16.8 400 100 71.6 12.0 400 100 51.4 17.8 400 81.0 20.4 19.6 400 68.5 18.6 22.1 400 18.1 25.4 16.0 MeOHc 0.2 35.3 0.2 23.9 0.4 24.4 1.9 17.4 0.8 7.8 5.9 12.1 MeOMed 5.7 21.9 5.4 25.4 5.6 20.5 tr 2.6 6.3 0.05 0.7 0.1 19.5 20.3 0.14 0.6 tr 13.3 20.0 0.70 0.7 tr 7.3 24.9 4.5 1.8 tr 5.4 2.4 - tr 6.2 6.6 - e - tr 11.8 - tr 15.8 8.4 0.8 0.3 tr 15.4 8.7 - a WHSV = g(Me0H) g(cata1yst)-' h-l. according to Olah.13 Based on total exit gas analysis.Catalyst prepared Catalyst prepared according to Maitra et aZ.22 Not determined. Table 2. Reaction of methylating agents over W03/A120, a hydrocarbon selectivity (YO by mass) conversion WHSV/h-l T/"C (Yo) Cl c2 c3 c, c,+ Me2S0, 0.05 300 2 92 8 tr - - 0.05 370 7 37 19 11 33 tr 0.03 400 21 82 11 4 3 tr 0.05 440 15 74 14 5 7 tr Me1 0.9 300 0.2 95 5 tr - - 0.9 400 8 97 2 1 tr - (MeO),PO 0.05 300 1 1 12 35 52 tr 0.05 350 4 81 5 5 9 tr 0.9 300 1 9 2 3 4 1 - " Test experiments showed the absence of blank thermal reaction for all reaction conditions given. a few hours. In contrast, H-ZSM-5 showed no decrease in activity over a similar reaction period. However, it has not been established that 10%-WO,/Al,O, is the optimum tormulation, and hence significant improvements may be possible with respect to both catalyst activity and lifetime.Reaction of Methylating Agents over WO,/Al,O, Me,SO, and Me1 were individually reacted over WO,/A1,0, using a range of reaction conditions, and the results are shown in table 2. These reagents were found to be much less reactive than MeOH or MeOMe, and the order of reactivity was similar to that observed for H-ZSM-5.l' At the low reagent conversions observed the hydrogen yield was low in all experiments,'l and for Me,S04 ranged from trace levels at 300 "C to 0.03 mol % at 440 "C. With both reagents the major product was methane, and methane1314 Hydrocarbon Conversion using WO,/Al,O, and H-ZSM-5 selectivity increased with increasing conversion. The formation of ethene and higher alkenes with Me,SO, is considered to be significant with respect to the mechanism of the C, -+ C , transformation, since under these reaction conditions formation of a trimethyloxonium ion from Me,SO, is not considered possible.'" To exemplify further that reagents incapable of trimethyloxonium formation can give rise to C,, hydrocarbons, (MeO),PO was reacted over WO,/Al,O,.The results (table 2) show that although a low conversion was effected a significant yield of C,, products were observed in addition to methane. Olah12* l3 has proposed that carbon-carbon bond formation with the WO,/Al,O, bifunctional acid-base catalyst occurs via a trimethyloxonium intermediate, and this mechanism has been termed the oxonium-ion ylide mechanism. Such an intermediate has also been proposed for super-acid catalysts, e.g.TaF,,,, and it has been extrapolated" that it is also involved in carbon-carbon bond formation with H-ZSM-5. However, in this study we have shown that carbon-carbon bond formation can occur with reagents, Me,SO, and (MeO),PO, which are unlikely to give a tri- methyloxonium ion intermediate. Whilst we have shown this previously for H-ZSM-5,Othe results of this study confirm that the onium-ion ylide mechanism, proposed by Olah12713 and van der Berg et aZ.25 is an unlikely possibility with WO,/Al,O,. The product selectivities for the formation of hydrocarbons from methylating reagents of the type Me-X (X = OH, I, OS0,Me) are considerably different for WO,/A1,0, and H-ZSM-5, particularly for C,, hydrocarbons.Fig. 1 shows the selectivities observed with these three reagents at comparable conversions. These vast differences in product selectivity may be indicative of different reaction mechanisms for these two catalysts. However, differences in C,, hydrocarbons can also be accounted for by the relative differences in Bronsted acidity for the two catalysts. For ZSM-5 the high Bronsted acidity gives rise to rapid secondary 27 that occur via carbocationic intermediates. Such mechanistic pathways are not so favoured with WO,/Al,O, and hence the C,, products tend to be the alkenes C,H, and C,H,. Reaction of Methanol over Model Catalyst Systems Methanol was separately reacted over y-Al,O, and SiO, and the results are shown in table 3. These two materials were chosen to model the separate behaviour of [AlO,] and [SiO,] tetrahedra in the ZSM-5 structure.Under all reaction conditions the products comprised almost exclusively methane and dimethyl ether, whilst only limited C-C bond formation was observed. These systems can be considered as models for the methane formation reaction over ZSM-5. Reaction of SiO, or y-Al,O, with methanol would lead to the formation of a surface methoxyl species. These species would preferentially react with the high localised concentration of methanol uia hydride abstraction to form methane, and via methylation to form dimethyl ether. Deprotonation via an adjacent oxygen site to form the methylide species considered responsible for C-C bond formation cannot occur to any appreciable extent. The surface coverages of WO, on a high surface area y-Al,O, for the 10%-W03/ Al,O, catalyst has previously been shown to be less than a mono1ayer.28 Hence in addition to y-Al,O,-supported WO, an appreciable surface area of y-Al,O, remains exposed.The results shown in table 3 indicate that any exposed y-Al,O, surface could contribute to the high methane yield observed with these catalysts. Additionally, unsupported WO, (Merck, P. A., calcined 550 "C in air, 16 h) was also found to be active for methane formation under reaction conditions comparable to those used for the WO,/Al,O, catalyst. The catalytic effect of pure WO, was short-lived, but that of y-Al,O, was observed for an extended reaction period. Comparison of the results for WO,/y-Al,O, with the pure components indicates that in combination the WO, and y- A1,0, exhibit a synergistic effect since the methane yield is significantly decreased and carbon-arbon bond formation becomes the dominant reaction. It is well k n ~ w n ~ ~ - ~ OG.J. Hutchings et al. 100 80 60 1315 - - ( b ) - 100 80 60 40 20 c, c2 c3 c4 c5+ 100 80 60 40 20 Cl c2 c3 c4 1 c5 40 20 100 80 60 40 20 i? Cl c2 c3 c5 100 80 60 40 20 100 80 60 40 20 Fig. 1. Comparison of product selectivities for reaction of reagents MeX (X = OH, I, OSO,, Me) over H-ZSM-5 and WO,/AI,O, at comparable conversions. catalyst reagent T/"C WHSV/ h-l conversion (%) (a) H-ZSM-5 MeOH 250 0.005 87 (b) H-ZSM-5 MeSO, 250 0.075 21 ( c ) H-ZSM-5 Me1 250 0.6 0.1 ( d ) W0,-AI,O, MeOH 400 0.008 97.5 (e) W0,-Al,O, MeSO, 400 0.05 21 cf) W0,-AI,O, Me1 350 0.09 1 that the structure of WO, is modified by y-Al,O,, and the results of this study demonstrate the catalytic significance of this with respect to methanol conversion.It is also apparent that the WO,/Al,O, catalyst requires further optimisation to eliminate the unwanted methane yield generated via reaction of methanol with the uncoated y-Al,O, surface. 44 F A R 11316 Hydrocarbon Conversion using WO,/Al,O, and H-ZSM-5 Table 3. Reaction of MeOH over WO,, AI,O, and SiO, at 400 "C hydrocarbon product reaction conversion to selectivity (mass YO) time /min MeOMe (CH,), c, c2 c3 c4+ 25 75 150 210 270 20 75 150 225 100 180 120 170 210 295 330 570 20 375 475 100 180 68.6 34.2 22.6 13.4 12.9 24.5 27.0 46.7 11.1 17.2 15.4 86.3 79.2 78.8 77.8 76.8 70.6 62.9 72.6 74.9 3.8 3.5 WO,," WHSV = 0.20 14.5 90.6 4.3 5.4 79.4 5.4 0.4 45.9 20.7 0.1 49.0 17.3 0.06 56.5 9.3 69.0 93.9 5.2 69.7 92.2 5.1 40.7 91.1 6.8 3.3 94.5 2.4 1 .o 88.6 4.8 0.1 71.5 10.3 3.4 95.1 2.1 3.6 96.6 1.6 3.2 96.7 1.8 3.4 97.4 1.8 4.6 98.1 1.9 2.7 97.1 2.2 22.5 96.6 0.9 17.3 97.7 1.0 14.6 98.1 0.8 0.06 68.5 11.4 0.04 74.7 10.0 WO,," WHSV = 0.075 WO,,b WHSV = 0.18 A1,0,, WHSV = 0.15 A1,0,, WHSV = 0.075 SiO,, WHSV = 0.22 1.6 3.9 30.8 33.7 34.2 0.5 0.4 0.6 2.2 6.6 18.2 I .8 1.8 1.5 0.9 tr 0.7 2.1 1 .o 0.9 20.1 15.3 3.5 11.3 2.6 - - 0.4 2.3 1.5 0.9 - - 1 .o - - - - - 0.4 0.3 0.2 tr tr " Merck, P.A. Prepared by calcination of ammonium metatungstate. Model Reaction for Methane Formation involving Hydride Transfer In our previous studiesz0 we have proposed that methane formation from methanol over the zeolite catalyst H-ZSM-5 involves transfer of a hydride from methanol to a surface methoxyl group.To model this proposal we have carried out the methanol conversion reaction in the presence of an excess of cycloheptatriene, a known hydride donor and a more potent hydride source than methanol. The results for both H-ZSM-5 and WO,/Al,O, are shown in table 4. Cycloheptatriene is not stable within the H-ZSM-5 catalyst; it preferentially reacts to give the isomerisation product toluene and is also methylated via reaction with methanol. However, in the presence of cycloheptatriene with H-ZSM-5 the methane yield is increased by ca. 50%, which is highly significant. Blank thermal reaction of cycloheptatriene did not yield any methane, and it is therefore concluded that the increase in methane observed can be ascribed to hydride donation from cycloheptatriene. These data therefore support the proposa120 for reaction of methoxyl with a hydride source.G.J . Hutchings et al. 1317 Table 4. Reaction of MeOH and cycloheptatriene (CHT) over W0,/A1203 and H-ZSM-5 at 400 "C product selectivity (mol %) time conversion hydro- toluene hydrocarbon product selectivity reaction total (% by mass) /min (%) MeOMe carbons CH4 'ZH4 C2H6 '3 '4 ' 5 , 35 225 350 460 35 170 205 300 20 60 100 155 30 85 130 180 96.0 94.8 92.3 87.1 98.5 100 100 100 100 100 100 100 100 100 100 100 49.8 53.3 60.6 63.6 17.1 13.2 9.3 15.8 - - - - - - - - WO,/Al,O,, MeOH" 50.2 - 81.1 3.4 46.7 - 87.7 2.4 39.4 - 88.6 2.2 36.4 - 88.0 2.4 24.6 58.3 70.1 3.3 29.0 57.8 77.0 2.5 28.1 62.6 78.7 2.4 27.1 57.1 79.3 2.4 100 - 0.76 17.8 100 - 0.76 16.8 100 - 0.78 17.4 100 - 0.79 15.9 H-ZSM-5, MeOH/CHTb9" 65.4 34.6 1.10 21.7 64.3 35.7 1.08 23.1 63.1 36.9 1.13 22.3 64.5 35.5 1.24 22.2 WO,/AI,O,, MeOH/CHT'vb H-ZSM-5, MeOH" 0.1 1.6 5.8 8.0 0.4 1.1 6.1 2.3 0.5 1.0 6.7 1.0 0.6 1.1 6.2 1.8 1.0 2.5 6.3 16.8 0.9 1.8 5.0 12.8 0.9 1.6 3.3 13.1 0.9 1.7 3.6 12.2 0.3 31.7 24.6 24.8 0.3 30.9 24.3 26.9 0.3 31.3 24.0 26.2 0.3 30.1 24.1 28.8 0.5 25.4 14.8 36.5 0.3 21.1 10.8 43.6 0.3 22.7 12.8 40.8 0.3 24.4 11.0 59.1 a WHSV = 0.2 h-l.CHT/MeOH = 1 : 1. WHSV = 0.1 h-l. With the WO,/Al,O, catalyst different results are obtained and a slight decrease in methane formation is observed in the presence of cycloheptatriene (table 4).These results are therefore indicative that methane formation with the WO,/Al,O, catalyst may not involve a hydride transfer process. Reaction of Methanol and Dimethyl Ether with Gas-phase Additives Hydrogen For both H-ZSM-5 and WO,/Al,O, hydrogen and methane are produced in comparable amounts, but whereas for H-ZSM-5 yields of methane and hydrogen are low, these products are dominant for WO,/Al,O,. To test whether the methane with both catalysts is formed via reaction of an intermediate with hydrogen, reaction of methanol over WO,/Al,O, and H-ZSM-5 was carried out in the presence of added hydrogen. The results, shown in fig. 2 and 3, indicate that addition of hydrogen to the reactant feed stream had no significant effect either on product selectivity or on conversion for a wide range of experimental conditions and methanol conversion.In particular, there is no increase in methane in these experiments. These results are strong evidence against the involvement of free methylene carbene, a mechanistic proposal cited in a number of studie~,~7 319 32 since an increase in methane yield would have been expected based on the known reaction between methylene carbene and hydrogen.33 44-21318 Hydrocarbon Conversion using WO,/AI,O, and H-ZSM-5 40 30 30 I e 20 20 10 10 conv. C, C, C, C, LO Y conv. C, C, C, ( d ) 40 1 . - 30 30 1 E 20 20 10 20 m c 4 conv. C, C, C, C4 Cl c, c, c 4 Fig 2. Reaction of MeOH over WO3/A1,O3 with (unshaded) and without (shaded) co-fed hydrogen (9.8 mol% added to carrier gas): (a) 275, (b) 300, (c) 350 and (d) 400 "C; WHSV = 0.08 h-'.100 80 60 2 40 20 E rn 100 80 60 40 20 conv. C, C, C, C4 conv. C, C, C, C4 100 100 80 80 3 60 60 P 40 40 20 20 v rn conv. C1 C, C3 C4 conv. C1 C2 C3 C4 Fig. 3. Reaction of MeOH over H-ZSM-5 with (unshaded) and without (shaded) co-fed hydrogen (9.8 mol YO added to carrier gas): (a) 250 "C, WHSV = 0.05 h-l; (b) 300 "C, WHSV = 0.05 h-l; (c) 370 "C, WHSV = 0.05 h-'; ( d ) 250 "C, WHSV = 1.7 h-'.G. J. Hutchings et al. 1319 Table 5. Reaction of MeOH and MeOMe in presence of NO reaction product selectivity (mass YO) WHSV NO time conversion reactant /h-' (mol YO) /min (%) CH4 C2H4 'zHfj '3 '4 ' 5 , Me,O 50 0 0 0 0 Me,O 50 1 .o 1 .o 1 .o 1 .o 0 Me,O 14.8 0 0 Me,O 14.8 3.0 3 .O Ob MeOH 0.24 0 5.0d 30 60 120 180 30 60 120 180 200 6 120 30 60 90 235 300 H-ZSM-5" 28.8 7.7 6.8 5.5 H-ZSM-5" 27.4 11.3 6.9 6.6 5.5 H-ZSM-5" 95.4 23.9 86.6 73.7 18.8 H-ZSM-5' 100 100 0.8 12.4 0.1 24.7 21.5 40.5 0.3 9.2 0.1 16.4 34.5 42.5 0.5 10.7 0.1 19.8 38.9 30.0 0.5 10.2 0.1 20.0 45.4 23.8 0.9 12.9 0.3 23.6 26.2 36.1 0.5 14.1 0.1 20.1 27.8 37.4 0.5 13.5 0.1 20.8 34.3 30.8 0.5 13.1 0.1 20.8 37.1 28.4 0.4 8.4 0.1 18.9 40.6 31.6 1.3 11.7 0.5 28.2 34.5 23.8 0.5 19.9 0.1 26.8 21.9 30.8 0.8 14.0 0.4 24.0 30.6 30.2 0.7 15.0 0.2 22.9 29.7 31.5 0.5 12.6 0.1 20.0 17.9 48.9 1.0 17.3 0.3 32.2 23.5 8.2 1.0 22.5 0.1 36.0 10.8 5.0 a 300 "C.NO addition stopped after 61 min. 400 "C. NO addition started 236 min. Nitric Oxide Recently a number of studies3,, 3 4 9 3 5 on the methanol conversion reaction have proposed that a radical pathway is the dominant mechanism for initial carbon<arbon bond formation.We have recently that the specific radical pathways proposed by Clarke et aZ.,32 i.e. involving rearrangements and reaction of the methoxymethyl radical, are not suitable pathways to ethene formation. However, to investigate the possible involvement of a general radical pathway, reaction of methanol and dimethyl ether over H-ZSM-5 and W03/Al,03 was studied in the presence of nitric oxide (table 5 and fig. 4). Nitric oxide, a monoradical, is well known as a radical scavenger3' at temperatures of up to 600 0C,38939 and for H-ZSM-5 it is observed that addition of 1 mol % NO had no effect either on catalyst activity or on product selectivity. Addition of 3 mol% NO only slightly enhanced catalyst deactivation, and no marked changes in product selectivity were observed.For the W0,/Al,03 catalyst, reaction in the presence of 5% NO did affect the overall catalyst activity and enhanced the methane selectivity significantly (fig. 5). The results of this investigation provide evidence that, for H-ZSM-5, a radical pathway is not involved in initial carbon-carbon bond formation. In addition, the production of methane relative to C,, products was also not affected and hence the study further confirms that methane is not generated via a radical methanism distinctly separate from the main carbon-carbon bond formation reaction. It must therefore be concluded that the radicals observed by Clarke et aZ32 in the reaction of dimethyl ether over H-ZSM-5 play no major role in the mechanism of methanol conversion.For1320 Hydrocarbon Conversion using W03/A1203 and H-ZSM-5 100 80 n E C .z 60 8 5 40 n 4 z 20 I 0 0 50 100 150 200 250 time on line/min Fig. 4. Reaction of MeOMe (A) and MeOMe/NO (0) over WO,/Al,O, at 400 "C and GHSV = 360 h-l: Effect on conversion. (Dashed line indicates time of no addition.) 80 n E 20 I I I - 0 I 1 I 1 1 I I I I 0 20 40 60 80 100 120 140 160 time on line/min Fig. 5. Reaction of MeOMe (A) and MeOMe/NO (0) over WO,/Al,O, at 400 "C and GHSV = 360 h-I: Effect on CH, selectivity. (Dashed line indicates time of no addition.) W03/A1203, however, the results indicate that a radical mechanism may play a significant role in carbon-carbon bond formation with this catalyst since addition of NO significantly reduces the formation of C,, hydrocarbons relative to methane.Oxygen The reaction of dimethyl ether and methanol in the presence of added gas-phase oxygen was investigated for both H-ZSM-5 and W0,/A1203. For W03/A1203 (fig. 6) addition of oxygen at concentrations of up to 20 mol% had no marked effect on activity or selectivity. Conversely, as published previously4o we have shown that for H-ZSM-5G . J . Hutchings et al. 1321 20 I 1 I I I 50 100 150 20 0 2 50 time on line/min Fig. 6. Reaction of MeOMe (A) and MeOMe/O, ( x ) over WO,/Al,O,. oxygen addition (1 and 3 mol %) causes immediate and irreversible deactivation of the zeolite catalyst, the effect being more pronounced at the higher oxygen concentratior,. These experimental results using oxygen addition are considered to be highly significant mechanistically and clearly demonstrate that the mechanistic pathways for C-C and CH, formation on the two catalysts are considerably different. Although oxygen is a diradical, such a species would not be expected to be a more effective radical scavenger than NO under these conditions.Triplet oxygen as well as nitric oxide, both stable free n-radicals, do readily react with organic radicals. The addition reaction between a radical and oxygen is usually followed by hydrogen abstraction to give a hydr~peroxide.~' Alternatively the peroxy radical intermediate might react with another radical to form a dialkyl Under the high temperatures used in the MTG process, however, peroxides and hydroperoxides will readily undergo homolytic cleavage,42 and thus the number of reactive open-shell species present in the system will not be reduced by 0,.NO, on the other hand, forms nitrosoalkanes on reaction with alkyl radical^^,.^^ which can tautomerise to an xim me.^^^^^ If a second a-hydrogen is present, the oxime might, under the acidic conditions within the zeolite, further undergo elimination of H20, forming a n i t ~ i l e . ~ ~ ' ~ ~ Therefore, NO is expected effectively to reduce the number of free radicals even at these high temperatures. Furthermore, the deactivation observed with oxygen addition was irreversible, and if oxygen were active as a radical scavenger such deactivation would have been expected to be (at least in part) reversible, and catalyst activity would be restored on elimination of the oxygen; this was not observed. We consider that the most likely explanation for this effect is that the crucial reaction intermediate in the C-C bond formation reaction with H-ZSM-5 is oxidised by molecular oxygen far more preferentially than by NO (only partial deactivation was observed with NO).However, for the WO,/Al,O, catalyst the crucial intermediate in the C-C bond formation is apparently not susceptible to oxygen attack. These results therefore present clear experimental evidence against the mechanistic proposals of Olah,127139 l7 in which dimethyloxonium methylide has been cited as the crucial intermediate with the catalyst WO,/Al,O,, since the ylide intermediate would be expected to be highly reactive towards molecular oxygen.471322 Hydrocarbon Conversion using WO,/Al,O, and H-ZSM-5 Table 6.Reaction of Me,O in presence of CH,O and HCO,H over H-ZSM-5 at 300 "C WHSV/h-l reaction Me,O product selectivity (% by mass) reactant Me,O CH,O CHO,H /min-' (YO) CH, C,H, C,H, C, C, C,, time conversion Me,O/CH,O/ 14.8 0.015 - 6 H20a 60 120 6 60 120 Me,O/HCO,Hc 14.8 - 1.55 6 60 120 Me,O/HCO,Hc 50 - 1.94 6 30 60 Me,0/H20b 14.8 - - 97.7 1.4 10.6 0.9 25.3 31.7 30.1 80.4 1.2 15.2 0.3 23.2 26.6 33.5 8.7 0.3 7.5 0.1 12.3 28.5 51.5 88.8 1.2 11.8 0.7 24.5 35.2 26.6 85.8 1.1 14.5 0.3 23.1 28.5 32.5 19.8 0.6 18.4 0.1 25.1 21.7 34.1 96.9 1.4 10.0 0.9 29.7 35.1 26.9 49.3 0.9 13.2 0.2 23.6 23.9 38.2 22.1 0.5 10.7 0.1 16.3 34.2 38.2 99.2 1.6 14.2 0.8 28.2 36.7 18.7 39.8 0.9 12.6 0.2 24.5 25.5 36.3 15.3 0.6 14.1 0.1 19.7 26.9 38.6 a Me,O bubbled through formalin.vapour pressure. Me,O bubbled through water to give equivalent water Me,O bubbled through formic acid, for comparable data see table 5. 100 80 n E 2 60 .* & > 0 40 01 I I I 1 1 0 50 100 150 200 250 time on line/min Fig. 7. Reaction of MeOMe (A) and MeOMe/HCHO (0) over WO,/AI,O,, 400°C and GHSV = 360 h-'. Reaction of Dimethyl Ether with Added Formaldehyde and Formic Acid Dimethyl ether was reacted over H-ZSM-5 in the presence of added formaldehyde and formic acid, and the results are shown in table 6. Formaldehyde addition was found to enhance catalyst deactivation even at low feed concentrations (ca. 0.1 YO by mass of dimethyl ether fed). Conversely, addition of formic acid did not demonstrate such severe deactivation even at feed levels of up to 10% by mass HCOOH relative to dimethyl ether.Dimethyl ether was reacted over WO,/Al,O in the presence of added formaldehyde and rapid catalyst deactivation was similarly observed (fig. 7).G. J . Hutchings et al. 1323 Discussion Comparison of Catalytic Activity of H-ZSM-5 and 10 % W03/A1,03 The principal difference in the reaction of methanol over H-ZSM-5 and W03/A1,03 is the relative yields of methane and hydrogen. For both catalysts these products are formed in roughly equal molar quantities, but for H-ZSM-5 these products constitute 1 % of the total product whereas for W03/A1203 methane and hydrogen are the dominant gaseous products. Experiments with the addition of hydrogen did not show any marked change in methane selectivity, demonstrating that hydrogen is not a precursor of methane formation with either catalyst.Since methane and hydrogen both increase by roughly the same factor with increasing temperature with W03/A1203 it is possible that they are formed via a common reaction mechanism. An additional major difference observed between W03/A1,03 and H-ZSM-5 is with respect to the conversion of the reactants. In all cases the W03/Al,03 is considerably less active than H-ZSM-5. This may be due to the high density of active sites within the zeolite catalyst4' compared to the oxide, or due to sequential autocatalytic reactions26, 27 that occur within H-ZSM-5, giving products of higher carbon number. Mechanism of Primary Product Formation H-ZSM-5 A large number of proposals have been made concerning the mechanism of formation of ethene,49 whereas only relatively 21 have considered the mechanism of formation of methane, which is a major primary product at low c o n ~ e r s i o n .~ ~ . ~ ' ~ ~ ~ Of the mechanistic proposals previously cited for ethene formation, only a few have received experimental support : ( a ) formation of carbene from methanol by a e l i m i n a t i ~ n , ~ . ~ ~ ~ ~ (h) intermolecular reaction of dimethyloxonium methy1ide,l2-l4* 17g51 (c) involvement of a radical pathway32,34,35 and ( d ) deprotonation of a surface bonded methyloxonium ion to give a surface-bonded oxonium methylide.1s*20~ 21 For H-ZSM-5 the experimental data obtained using model reagents and catalyst systems in this and our previous ~tudies'*-~~~ 3 6 9 4 9 9 52 presents clear and strong evidence against the involvement of pathways (a), (6) and ( c ) noted above.In particular, our experiments with model catalysts5, and model reagent^'^?^^ showed that pathway (b) was not viable, and the arguments need not be reported in this discussion. We have previously shown2' that proposal (a) cannot account satisfactorily for methane formation. Experiments with co-feeding hydrogen further demonstrate that free gas- phase methylene carbene is not a central intermediate, since no increase in methane selectivity was involved. This conclusion is supported by the experimental data obtained for the co-feeding of oxygen. Reaction of a gas-phase methylene intermediate with molecular oxygen is known to produce formic acid via a Criegee inte~mediate.~~ Formic acid does not deactivate H-ZSM-5, and hence if gas-phase methylene carbene were to be the central intermediate in carbon-carbon bond formation, reaction with oxygen would not lead to permanent deactivation of the catalyst, and this is not experimentally observed.We have previously shown that radical pathways involving the methoxy methyl radical are not viable for C-C bond f ~ r m a t i o n . ~ ~ In addition, the present study using experimentation with the known radical scavenger NO demonstrates that for H-ZSM- 5 neither C-C bond formation nor CH, formation involve a radical pathway. Additionally Chang54 has recently proposed that a surface CH, radical could be generated via interaction of a radical initiator with a surface methyloxonium ion. However, the results obtained with NO would also mitigate against such a mechanistic proposal.1324 Hydrocarbon Conversion using WOJ A1,0, and H-ZSM-5 CH4 (b) C H j + H' - / (c) CH3' + H- Fig.8. Possible mechanisms for the formation of methane in the methanol conversion reaction. Based on our previous studies with methylating agents", 21 we proposed a methylation mechanism for H-ZSM-5 conversion of methanol in which a surface-bonded methyloxonium ion is the first key intermediate in the process. This species is formed oia nucleophilic attack of lattice zeolite oxygen at the carbon centre of protonated methanol which we have modelled by the reactions of LiAl(O-Pri),.52 Furthermore, methylation of H-ZSM-5 has been observed e~perimentally.~'~~~ Recently, Forester and Howe", 57 have provided evidence using in situ Fourier-transform infrared studies that formation of a surface bonded methyloxonium ion precedes carbon-carbon bond formation with H-ZSM-5.This methyloxonium ion is not responsible itself for initial carbon-carbon bond formation, but is the intermediate responsible for CH, formation and further methylation of the initial products.26v58 Formation of methane is considered to be highly significant mechanistically. Methane can only be formed via three mechanistic pathways during methanol conversion (fig. 8). In this study we have shown that methane formation from gas-phase carbene and methyl radical intermediates is not consistent with the experimental evidence. In addition, using a model study with the known hydride donor cycloheptatriene we have successfully modelled methane formation via H- donation, hence supporting the proposal that the surface-bonded methyl oxonium ion is the central intermediate in methane formation.We have p r o p o ~ e d l ' ~ ~ ~ that the key intermediate in the formation of the initial C-C bond is a surface bonded oxonium methylide formed via deprotonation of the surface methyloxonium ion. Recent experiments on methanol oxidation with molybdate catalystP have also postulated a deprotonation of a surface-bonded methoxyl to form a surface-bonded oxonium methylide as a key intermediate in formaldehyde formation. Evidence for this deprotonation step was obtained using kinetic-isotope data.5Y The experimental data obtained on the reaction of MeOH/O, over H-ZSM-5 provide evidence in favour of the existence of surface-bonded oxonium methylide as the key intermediate, and are therefore consistent with the oxidation of this intermediate to form formaldehyde, which we have subsequently shown to deactivate the catalyst at very low concentrations.From a mechanistic point of view it has always seemed attractive to us that the surface-bonded methyloxonium ion, in harmony with its adjacent conjugate base, should interact with methanol in a synergistically intimate fashion to effect C-C bond formation. Hence we envisage that the conceptually demanding prospect of a naked, surface-bonded methyloxonium-ion ylide acting as a nucleophile may be facilitated by considering that proton abstraction in the transition state by an adjacent alu- minium-oxygen bond renders the methanol carbon more accessible to nucleophilic attack by virtue of hydrogen bonding between the hydroxyl oxygen and the hydrogen being transferred (fig.9). Thus proton abstraction could well be enhancing activation of the poorly electrophilic methanol carbon in the tightly bound pocket of the zeolite channel. We consider that a radical hydrogen abstraction to generate an oxonium ion radical intermediate, although exciting as a possibility, is discounted in its extreme form by the NO co-feeding results. However, we recognise that serious thought as to the full implications of the two mechanistic extremes poses fascinating, if not daunting challenges on both the theoretical and synthetic fronts.In conclusion, the experimental data presented in this paper support the methylationG . J. Hutchings U et al. /*\ 1325 H-ZSM-5 Fig. 9. Mechanistic proposal for C-C bond formation over H-ZSM-5. mechanism previously proposed by us1** 21 and provide clear evidence against alternative mechanistic proposals. Future studies are now required to model successfully the deprotonation step in this mechanism. The experimental results from reaction of Me,O/O, and Me,O/NO over WO,/Al,O, differ significantly from those obtained for H-ZSM-5, and it is proposed that a different mechanism exists for this catalyst system. Of particular note are the results using Me,O/O,, which suggest that the mechanistic pathway is insensitive to the addition of up to ca.20 mol % molecular oxygen. This presents clear evidence against the involvement of a non-surface-bonded dimethyloxonium methylide as proposed by Olahl29 13, l7 specifically for this catalyst, since such an ylide intermediate would react rapidly with oxygen47 to inhibit the C-C bond formation reaction. The result also mitigates against a surface-bonded oxonium methylide intermediate as proposed for H-ZSM-5. Again the origin of methane in this reaction is considered to be mechanistically significant. The evidence presented in this study indicates that methylene carbene [fig. S(a)] and hydride transfer [fig. 8 (c)] are not viable possibilities, since reaction with co-fed hydrogen (fig. 2) or co-fed cycloheptatriene (table 4) have no significant effect. In the light of the evidence presented the following mechanism is proposed for methanol conversion over WO,/Al,O, (fig.10). Methanol dissociatively adsorbs onto the tungsten surface to form surface methoxyl and hydroxyl groups. In this reaction methanol acts as a nucleophile towards the electrophilic WO, centre, so that the oxygen of the methoxyl group is retained from methanol. This is considerably different from the electrophilic behaviour of protonated methanol towards the conjugate base of H-ZSM- 5. Methane may be generated from the hydroxyl and methoxyl surface groups by a dissociative H/CH, recombination pathway. This possibility is not open to the ZSM-5 system, and is considered to account for the drastic difference in methane levels between the two catalysts. The reaction, in giving rise to surface-bonded radicals, consequently1326 Hydrocarbon Conversion using WO,/Al,O, and H-ZSM-5 H o/cH3 \O - H 2 (adjacent OH) H migration CH3OH * PNO 1 adjacent methyl "2'4 \ + CH2O deactivation p", \ /CH2-cH3 W - P/ O-3- /I Fig.10. Proposed mechanism for methanol, dimethyl ether conversion of form C,H, and CH, over WO,/Al,O,. allows mechanistic appraisal of two key processes, namely C-C bond formation and surface deactivation. The surface oxygen radicals may abstract hydrogen from methanol in a radical redox process to generate formaldehyde which results in irreversible deactivation as is experimentally observed. Alternatively they may abstract hydrogen from an adjacent neutral surface methoxyl to furnish a methyleneoxy radical which by radical attack on either another adjacent methoxyl, or less likely methanol itself, leads to C-C bond formation.Co-feeding NO, while inhibiting radical C-C bond formation, would not, according to the present rationale, be expected to affect CH, formation, since the latter is formed in a non-radical-dependent pathway. Indeed this is what is experimentally observed, with C,, product selectivity decreasing dramatically and CH, selectivity increasing under these conditions, Assuming that co-feedants interact with the surface as radicals, as discussed previously, the difference in reactivity between them for this catalyst may be rationalised in terms of considering the products of radical scavenging. Whilst 0, results in formation of a new radical which may sustain chain growth, NO results in chain termination.Such a drastic change in mechanistic behaviour for the two catalysts would be due primarily to the electronic nature of the oxygen bonded to the methyl group in each case. Hydrogen, a significant by-product in this reaction, may also be produced by a radical pathway, favoured at higher temperatures. The role of the exposed A1,0, surface known to be present in this catalyst system is unclear at this stage. However, its influence is obviously essential for C-C bond formation, which is virtually non-existent with either WO, or AI,O, alone.G. J. Hutchings et al. 1327 Deactivation of ZSM-5 during Methanol Conversion The mechanism of deactivation of zeolite ZSM-5 during methanol conversion has not received much attention.It is known that by virtue of its shape selectivity deactivation due to coke, a highly carbonaceous deposit, is restricted with ZSM-5 compared to other Solid state magic-angle spinning n.m.r. analysis of the coke indicates that it contains aromatic, aliphatic and oxygenated ether and ketonic carbon environments.61~ 62 In the present study reaction of MeOMe/HCHO mixtures over H-ZSM-5 has been shown to deactivate the catalyst rapidly, and additionally when MeOH/O, or MeOMe/ 0, are reacted rapid deactivation is proposed to result from formation of HCHO at the active site. It is therefore probable that some deactivation of H-ZSM-5 occurs during methanol conversion owing to HCHO formation and polymerisation reactions. The resulting formation of highly oxygenated coke residues would be consistent with the previously cited magic-angle spinning n.m.r.evidence. In our previous proposalsz1 we have concluded that HCHO would be formed via hydride transfer from methanol to the surface methyloxonium ion during CH, formation. Catalyst deactivation would therefore be proportional to CH, formation over H- ZSM-5; however, this situation would be complicated by the laydown of aromatic-type coke owing to the rapid secondary reactions occurring on this catalyst. The results of this study indicate that low levels of 0, could be significantly deleterious in the commercial operation of ZSM-5 for methanol conversion. Since zeolites are reactivated on a regular basis with 0,-N, mixtures it is particularly important that the effect of 0, in catalyst lifetime be more fully examined. We thank the University of the Witwatersrand and the FRD, CSIR Pretoria for Financial Support.References I G. J. Hutchings, New Scientist, 1986, (3rd July) 35. 2 A. V. Grosse and J. C. Snyder, US Patent 2492984 (1950). 3 L. Kim, M. M. Wald and S. Brandenberger, J. Org. Chem., 1978, 43, 3432. 4 D. C. Hargis and L. J. Kehoe, US Patent 4072732 (1978). 5 D. E. Pearson, J. Chem. SOC., Chem. Commun., 1974, 397. 6 C. A. Fyfe, G. C. Gobbi, J. Klinowski, J. M. Thomas and S . Ramdas, Nature (London), 1982, 296, 7 S. L. Meisel, J. P. McCullough, C. H. Lechthaler and P. B. Weisz, Chemtech., 1973, 3, 498. 8 C. D. Chang, and A. J. Silvestri, J. Catal., 1977, 47, 259. 9 C. D. Chang, Catal. Rev. Sci. Eng., 1985, 26, 323. 10 C.T-W. Chu and C. D. Chang, J. Catal., 1984,86, 297. 11 M. M. Wu and W. W. Kaeding, J. Catal., 1984, 88, 478. 12 G. 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SOC., 1987, 109, 5076. 58 T. Mole, in ‘Proc. Methane Conversion Symposium’, Auckland, New Zealand, April 1987, ed. D. M. 59 C. J. Machiels and A. W. Sleight, J. Catal., 1982, 76, 238. 60 P. Dejaifre, A. Auroux, P. C. Grabelle, J. G. Vedrine, Z. Gatelica and E. G. Derouane, J . Card., 1985, 61 E. G. Derouane, J. P. Gilson and J. B. Nagy, Zeolites, 1982, 2, 42. 62 L. C. Carlton, R. Copperthwaite, G. J. Hutchings and E. C. Reynhardt, J. Chem. SOC., Chem. L. V. C. Rees (Heydon, London, 1980), p. 649. R. T. K. Baker, in Solid State Chemistry in Catalysis, ACS Symp. Ser. 279, 1985, p. 165. 559. J. Chem. SOC., Chem. Commun., 1986, 425. p. 1 . and P. G. Sammes, J. Am. Chem. SOC., 1965, 87, 4601. (Wiley, New York, 1983), part 2, pp. 1345-1390. Bibby, C. D. Chang, R. F. Howe and S . Yurchak (Elsevier, Amsterdam, in press). Bibby, C. D. Chang, R. F. Howe and S. Yurchak, (Elsevier, Amsterdam, in press). 70, 123. Commun., 1986, 1008. Paper 71023; Received 5th January, 1987

ISSN:0300-9599    

DOI:10.1039/F19888401311

出版商:RSC    

年代:1988

数据来源: RSC

摘要:

J . Chem. SOC., Faraday Trans. I, 1988, 84(5), 1329-1340 A New Ti0,-attached Rhodium Metal Catalyst Catalyst Characterization and Non-SMSI Behaviour Kiyotaka Asakura, Yasuhiro Iwasawa" and Haruo Kuroda Department of Chemistry, Faculty of Science, The University of Tokyo, Hongo, Tokyo 113, Japan Ti0,-attached Rh metal cluster catalysts have been prepared by the reaction between Rh(v3-C,H,), and surface OH groups of TiO,, followed by H, treatments at different temperatures. This class of Rh/TiO, catalysts, which are very active for ethene hydrogenation and ethane hydrogenolysis, show no SMSI phenomena, unlike conventional Rh/TiO, catalysts prepared by a traditional impregnation method using aqueous RhCl,. The structural properties of these catalysts, such as a shortening of the Rh-Rh distance, their raft-like structure and their positively charged character, have been studied in relation to non-SMSI behaviour by means of EXAFS together with other physiochemical techniques.These properties of structure, electronic state, chemisorption and catalytic activity may be ascribed to the Rh-0-Ti bonds which are formed in during attachment of the Rh-ally1 complexes onto TiO, and are partially retained after high-temperature H, reduction. Transition-metal n-ally1 complexes readily react with surface OH groups of inorganic oxides such as SiO,, A1,0, and TiO, to form attached-metal catalysts with well defined structures and properties on a molecular level as well as to provide highly dispersed metal particles on the support surfaces by the subsequent reduction with H, under mild conditions.'* The attached noble-metal catalysts such as Pd/Si0,,3,4 Pt/Si0,,4 Rh/Si0,,5,6 Rh/Ti0,7 etc.have been prepared by this method. TiO, has attracted much attention as a support because the 'SMSI effect' appears when H, or CO are chemisorbed on noble metals when they are dispersed on TiO,.' We have prepared a Ti0,-attached Rh catalyst using Rh( r73-C3H5)3,7a which shows higher activities for ethane hydrogenolysis and ethene hydrogenation than a con- ventional (impregnated) Rh/TiO, catalyst.'" In addition to this difference in activity, they also exhibit different adsorption behaviour ; the impregnated Rh/TiO, catalyst almost loses the ability to chemisorb H, and CO after high-temperature reduction with H,(SMSI phenomenon),8 while the Ti0,-attached Rh catalyst does not.T.e.m. studies have shown that the average size of Rh particles in the attached catalyst is ca. 1.4 nm, i.e. smaller than the Rh particles prepared by the impregnation method (ca. 3.1 nm). The uniqueness of the present attached sample is not due to the small particle size because Haller et al. pointed out that the SMSI effect is more apparent for smaller Rh particle^.^ A different metal-support interaction should be considered in this attached Rh/TiO, system, which would reflect the bonding feature and electronic state of the Rh metal particles on TiO,. It has been demonstrated by many researchers that EXAFS is a powerful tool for investigating the local structure around a specific atom in supported fine metal particles." It is potentially possible to elucidate the metal-support Recently Rh/TiO, catalysts prepared by an ion-exchange method were studied by means of EXAFS and direct Rh-Ti bonding was observed in the SMSI state.14*15 XANES is 13291330 New Ti0,-attached Rh Catalyst also powerful in obtaining information about the electronic state of metal atoms.In the present work, the bonding features and electronic states of the Rh atoms in the Ti0,- attached Rh catalysts were studied by means of EXAFS and XANES together with other physicochemical techniques such as i.r., t.e.m. and temperature-programmed decomposition (t .p.d.). Experimental Preparations and Treatments of the Catalysts The attachment procedure of Rh(q3-C3H,), onto TiO, (partially dehydrated Degussa P-25) is illustrated in scheme 1 .7 a The TiO, surface was first decarboxylated by heating to 790 K, exposed to water vapour at 293 K, then treated at 473 K under vacuum in situ A B Scheme 1.The attachment of Rh(v3-C3H5), onto TiO, and surface transformations. before the attachment reaction. The attachment reaction was carried out at 273 K in a specially devised Pyrex rea~tor.~' The Rh loading was 2.0 wt YO as Rh/TiO,. The attached Rh species (species A) was treated with H, to form species B at 293 K. B was reduced with H, at 773 K, followed by oxidation with 0, at 673 K. Then the sample was reduced with H, at the desired temperatures. The first reduction-oxidation process was carried out to minimize the effect of aggregation of the Rh particles. For comparison, the Si0,-attached Rh (2.0 wt YO) catalysts were prepared in a similar manner using the reaction between Rh(q3-C,H,), and the pretreated SiO,.The impregnation Rh/Ti0,(2.0 wt%) catalysts were obtained by the usual impregnation method using an aqueous RhCl, solution, followed by calcination at 673 K. The impregnation catalysts were reduced with H, at 473 and 773 K and are denoted as I-Rh/Ti02(473) and I-Rh/Ti02(732), respectively. Surface Characterization of the Samples 1.r. spectra were recorded in an i.r. cell connected to a closed circulating system on a JASCO IR-8 10 spectrometer. T.p.d. measurements were carried out by heating the samples at a rate of 4 K min'l. The evolved gases were analysed by gas chromatography. EXAFS spectra were measured at the beam line 10B of the Photon Factory (KEK-PF).l' The samples treated in a closed circulating reactor were transferred to EXAFS measurement cells which were also connected to the same closed circulating reactor.Thus air contact was avoided completely. T.e.m. photographs were taken on a JEOL-200B electron microscope. Results and Discussion Surface Structures of Species A and B Fig. 1 shows the i.r. spectra of species A and B. The absorption bands of species A are observed at 3050 and 1462 cm-l, suggesting that the allyl ligand is still of x-ally1 character.,., After the exposure of A to H, at 9.3 kPa for 1 h at room temperature, the allyl ligand was completely removed and a new peak appeared at 2048 cm-l, as shownK. Asakura, Y. Iwasawa and H. Kuroda 1331 I 1 I 3000 2000 1500 wavenumber/crn-' Fig.1. 1.r. spectra of the species A(a) and B(b). The broken curve of (b) shows the species after deuterium exchange of species B. Table 1. Stoichiometries in attachment of Rh(q3-C3H,), and surface transformations. amount of Rh attached on TiO, 2.0 wt Yo no. of allyl ligands per Rh atom from t.p.d. 0.98 from H, reaction 1.10 in fig. 1. Gas-phase analysis showed that 1.10 molecules of H, per Rh atom were consumed in the reaction of the allyl ligands with H, as given in table 1. The i.r. peak observed with B shifted to 1495 cm-l after deuterium exchange, as shown in fig. 1. Thus, this peak is attributed to the Rh-H vibration of terminal hydride species B. No peaks corresponding to bridging hybrides or the allyl group were found for species B, unlike the previously reported SiO, 5y6'and Ti0,-attached catalyst^.^ Fig.2 shows the t.p.d. spectrum of species A.17 The main products of t.p.d. were C,H6 and C,H,. The number of allyl ligands per Rh atom in species A was determined from the t.p.d. chromatogram to be 0.98, in good agreement with the result obtained from the reaction with H, as shown in table 1. From these results the amount of the allyl ligand in species A is unity per Rh atom. The production of C,H, in fig. 2 seems to be unusual, but we infer that the C,H, peak may be caused by the interaction of the allyl ligand with a Ti surface atom as previously reported." From i.r., t.p.d. and H, titration, the incipient surface Rh complex and subsequent Rh-hydride species may be illustrated as in scheme 1.Chemisorption Measurements Fig. 3 shows the results of H, chemisorption on the Rh/TiO, and Rh/SiO, catalysts. The A-Rh/SiO, catalysts showed 100% dispersion (H/Rh = I), all Rh atoms being available for H, adsorption. The dispersion of ca. 50 % for the A-Rh/Ti02(473) catalyst is still high as compared with 24 YO dispersion for the I-Rh/Ti02(473) catalyst. From the dispersion of the catalysts and the assumption that the shape of the Rh particle is spherical and that all surface Rh atoms are available for H, chemisorption to form Rh-H bonds, the Rh particle sizes (diameters) are estimated to be 2.9 and 6.0 nm for the attached and impregnated Rh/TiO, catalysts, respectively. The I-Rh/TiO, system largely lost the ability to adsorb H, after high-temperature reduction owing to the so-1332 New 730,-attached Rh Catalyst 273 473 t T/K 13 Fig.2. Temperature-programmed decomposition of the species A ; (---) C,H,, (- - -) C,H,, (-) c H , ( ...... ) C3H8 and (-. --) C,H8. 0 -0.5 \ W x w 4 - 1 . 0 - 1 . 5 ‘b 1 I I 1 473 573 673 773 873 T/K Fig. 3. H, chemisorption of the attached and impregnated Rh/TiO, and Rh/SiO, catalysts; 26.6 kPa H,; (a) A-Rh/SiO,, (6) I-Rh/SiO,, ( c ) A-Rh/TiO, and ( d ) I-Rh/TiO,.J . Chem. SOC., Faraday Trans. 1, Vol. 84, part 5 Plate 1. (a) and (b), for legend see following page. K. Asakura, Y. Iwasawa and H. Kuroda Plate 1 (Facing p . 1332)J . Chem. Soc., Faraday Trans. 1, Vol. 84, part 5 Plate 1. (c) and ( d ) , for legend see facing page. Plate 1 K. Asakura, Y. Iwasawa and H. KurodaJ .Chem. SOC., Faraday Trans. 1, Vol. 84, part 5 Plate 1 Plate 1. The TEM photographs of (a) TiO,(P-25), (b) A-Rh/Ti02(773), (c) A-Rh/Ti02(863), ( d ) I-Rh/Ti02(773) and (e) A-Rh/Si02(773). The magnification is 450000 x . Some Rh particles are indicated by circles and arrows. K. Asakura, Y. lwasawa and H. KurodaK . Asakura, Y. Iwasawa and H . Kuroda 1333 ~ 6 12 L 18 24 i L S i2 18 24 particle size/ lo-’ nm d 2 3 4 5 r (d 1 1 3 4 5 I tFj-li-ll particle size/nm Fig. 4. Particle-size distribution of Rh metal in the A-Rh/TiO, and I-Rh/TiO, catalysts determined from t.e.m. ; (a) A-Rh/Ti02(473), (b) A-Rh/Ti0,(773), (c) I-Rh/Ti0,(473) and (d) I-Rh/Ti02(773). Average particle diameters are 1.4 nm (a), 1.5 nm (b), 3.1 nm (c) and 3.4 nm (4. called ‘SMSI effect’, as shown in fig.3. In contrast to the traditional I-Rh/TiO, catalyst, the attached Rh catalyst showed no SMSI phenomenon, the dispersion remaining 40-50% in the reduction temperature range 473-773 K. The decrease in dispersion observed with A-Rh/Ti02(873) may be due to the sintering of metal particles as proved by t.e.m. measurements. T.E.M. Measurements Plate 1 shows micrographs of TiO,(P-25), A-Rh/Ti02(773), A-Rh/Ti02(873), 1-Rh/TiO2(773) and A-Rh/Si02(773). The Rh particles of A-Rh/Ti02(773) [shown by circles and arrows in plate 1 (h)] were very small compared with those of I-Rh/Ti02(773) in plate 1 ( d ) . The H, reduction of A-Rh/TiO, at 873 K led to an increase in the particle size as shown in plate 1 (c), which gives an Rh size of 1.9 nm on average.However, the Rh particles were resistant to agglomeration on reduction with H, up to 773 K. The particle size distributions were obtained by t.e.m. analyses in fig. 4. The average diameter of Rh particles was estimated to be 1.4 and 1.5 nm for A-Rh/Ti02(473) and A-Rh/ Ti0,(773), respectively. On the other hand, the average diameters were 3.1 and 3.4 nm for I-Rh/Ti02(473) and I-Rh/Ti02(773), respectively. The particle-size distributions were much narrower for the attached catalysts than for the impregnated catalysts, implying that the Rh atoms prepared from the attachment of Rh(q3-C3H,), via the1334 New Ti0,-attached Rh Catalyst h Y .r( 7 0 0.1 0.2 0.3 0.4 0.5 rlnm Fig. 5. Fourier transform of the Rh K-edge EXAFS spectrum of the attached Rh species (B) formed from the attached Rh species (A).chemically bonded surface species A (scheme 1) constitute relatively homogeneous and well dispersed small particles. The high dispersion of Rh metal in the A-Rh/SiO, catalyst is shown in plate 1 (e). The average size of Rh particles of A-Rh/Si02(773) was 1.3 nm. The impregnated Rh/Si0,(773) catalyst showed a wide distribution of Rh particle size (1 .O-5.9 nm) with an average diameter of 2.8 nm. Thus, well dispersed Rh metal with a narrow particle-size distribution appears to be formed from the surface atomic Rh species with the covalent bonds of Rh-0-Ti (-Si). EXAFS In order to investigate the local structures of Rh metal particles in A-Rh/TiO, and I-Rh/TiO,, we performed EXAFS analyses for these Rh catalysts during the preparation processes.We extracted EXAFS oscillation p ( E ) from the observed data by subtracting the smoothly varying part ,us(E) which was estimated from the cubic spline method." The oscillation thus obtained was normalized by the absorption coefficient of the free atom (1) Po(E) X(k) = ME) - , u s ( ~ ) I / P o ( E ) where k is the photoelectron wavenumber related to E by k = [2m/h2(E-EO)]t. (2) Eo is the threshold energy and is usually taken to be equal to the energy of the inflection point for convenience. We performed Fourier transformation of k3-weighted ~ ( k ) over the region 40-120 nm-l as shown in fig. 5 and 6. The peak in the Fourier transformation was filtered and then inversely Fourier- transformed back into k-space. The Fourier-K. Asakura, Y. Iwasawa and H.Kuroda 1335 4 6 8 10 12 k/ 10 nm-' 4 n Y x o .y - 4 I I I I 4 6 8 10 12 k/ 10 nm-' 8 n Y * o X - 8 I 1 I I I 4 6 8 10 12 k/10 nm-' r/ lo-' nm 0 1 2 3 1 5 r/ lo-' nm Fig. 6. EXAFS oscillations k ~ ( k ) and their Fourier transform (FT) of (a) A-Rh/Ti02(773), (b) A-Rh/Ti02(473) and (c) Rh metal. filtered data [X'(k)J were then analysed by means of a least-squares curve-fitting method using the theoretical EXAFS equation :la ~ ' ( k ) = SNF(k) exp ( - 2a2k2) sin [2kr + $(k)]/(kr2) (3) where N, Y and o represent the coordination number, bond length and Debye-Waller factor, respectively, these values being fitting parameters, together with Eo. $(k) and F(k) are the phase shift and amplitude functions, respectively, for which we used theoretically derived va1~es.l~ S is the amplitude reduction factor which arises from many-body effects and inelastic losses in the scattering process.This value can be regarded as a constant function of k because many-body effects and inelastic losses have opposite k- dependence.'O The results are given in tables 2 and 3. The S values of Rh-Rh and Rh-0 were determined from Rh metal and Rh,O,, chosen as reference materials. The Rh-Rh distance of bulk Rh metal thus determined was 0.266 nm, while 0.268 nm has been reported from X-ray crystallography. Fig. 5 shows the Fourier transform of an Rh1336 New Ti0,-attached Rh Catalyst Table 2. Curve-fitting results for Rh-0 (and Rh-C) peaks in the EXAFS data of the attached Rh species sample coordination no. distance/nm DW factor/nm A 3.0 (0.3) 0.214 (0.002) 0.0076 (0.001) B 2.0 (0.3) 0.2 15 (0.002) 0.0065 (0.001) Table 3.The curve-fitting analyses of the EXAFS data for the reduced Rh metal catalysts sample coordination distance DW factor edge energy no. /nm /nm / e v A-Rh/Ti02(473) 7.7 (2.0) 0.262 (0.002) 0.0081 (0.0010) 4.0 (0.5) A-Rh/Ti02(773) 8.3 (2.0) 0.262 (0.002) 0.0083 (0.0010) 3.1 (0.5) I-Rh/Ti02(473) 12.0 (2.0) 0.266 (0.002) 0.0076 (0.0010) 0.4 (0.5) I-Rh/Ti02(773) 12.0 (2.0) 0.267 (0.002) 0.0078 (0.0010) 0.4 (0.5) A-Rh/Si02(473) 5.0 (2.0) 0.264 (0.003) 0.0089 (0.0010) 0.0 (0.5) A-Rh/Si02(773) 5.2 (2.0) 0.264 (0.003) 0.0087 (0.0010) 0.0 (0.5) Rh metal (12) 0.266 (0.002) 0.0040 (0.0010) 0.0 Errors are indicated in the parentheses. K-edge EXAFS spectrum for the attached Rh species B, which was obtained by the interaction of the v3-allyl-type surface species A with H, at 298 K for 1 h.Since the scattering power of hydrogen is very small, the Rh-H bond does not contribute to the EXAFS oscillation. Only one peak assignable to the Rh-0 bond was observed at a distance of 0.215 nm (table 1) and no peak for the Rh-Rh bond was observed in the range 0.1-0.5 nm, as shown in fig. 5. This suggests that the Rh atom of species B is monoatomically distributed on the TiO, surface. The coordination number was determined to be 2 for Rh-0 bonds, which supports the bonding of Rh atoms to the surface through the oxygen atoms in a bidentate form as shown in scheme 1. Aggregation of the metal particles does not occur in this stage. This behaviour is different from that of the Si0,-attached Pd, Pt and Rh ally1 complexes, in which the metal aggregated during room-temperature r e d u ~ t i o n .~ * ~ Table 2 also shows the bond lengths and coordination numbers of Rh-0 (and Rh-C) bonds of the Ti0,-supported Rh-ally1 species A determined by curve-fitting analysis of the EXAFS data. The three Rh-C bond lengths for an q3-allyl ligand in (DBM)Rh(C,,H,,) are known to be 0.21 1, 0.218 and 0.221 nm,2* i.e. close to the Rh-0 bond length. Thus EXAFS cannot distinguish between the bond lengths of Rh-C(q3-allyl) and Rh-O(Ti0,). Therefore, a one-shell fit analysis was performed. Since species A was determined to have one $-ally1 ligand per Rh atom by i.r., t.p.d. and chemical analysis, the total coordination number of the carbon and oxygen around an Rh atom would be expected to be five.However, the large deviation of the bond length of Rh-C(q3-allyl) might diminish the contribution of Rh-C bonding to the total coordination number owing to the large static disorder. Fig. 6 shows the k-weighted EXAFS oscillation [ k ~ ( k ) ] and the Fourier transforms of A-Rh/Ti02(473), A-Rh/Ti02(773) and Rh metal. The oscillation observed in the higher wavenumber region implies the presence of Rh-Rh bonds and a peak around 0.25 nm (phase-shift uncorrected) assignable to an Rh-Rh bond was observed in each Fourier transform. The peak was further analysed by a curve-fitting technique. The results are given in table 3. The Rh-Rh bond lengths of I-Rh/Ti02(473) and I-Rh/Ti02(773) were 0.266nm, the same as the value for Rh metal.However, the A-Rh/Ti02(473) and A-Rh/Ti02(773) catalysts were found to have a bond length of 0.262 nm, which isK. Asakura, Y. Iwasawa and H . Kuroda 1337 shorter than those of Rh metal and the I-Rh/TiO, catalysts. The bond lengths of A-Rh/Si02(473) and A-Rh/Si02(773) were intermediate. The coordination numbers of A-Rh/Ti02(473) and A-Rh/Ti02(773) were determined to be 7.7 & 2.0 and 8.3 2.0, respectively, by EXAFS analysis. On the other hand, the Rh metal particles of both I-Rh/TiO, catalysts showed the same coordination number (12 k 2.0) as that of bulk Rh metal. Note that A-Rh/Si02(473) and A-Rh/Si02(773) showed very small coordination numbers, 5.0 & 2.0 and 5.2 k 2.0, respectively. The structure (morphology) of the Rh particles attached on TiO, can be discussed from the coordination number of Rh(EXAFS) and the particle size (t.e.m.) by the method reported by Greegor and Lytle.,, In this analysis only the first shell was used because the higher shells were too small to obtain the accurate coordination number, N i / N , values (Ni is the average coordination number of the first shell determined from EXAFS and N , is the bulk coordination number) were calculated to be 0.64k0.17 and 0.69 4 0.17 for A-Rh/Ti02(473) and A-Rh/Ti02(773), respectively, and the particle sizes were ca. 1.4 nm.According to the method of Greegor and Lytle, Ni/Nl should be ca. 0.89 in the case of a spherical particle with a diameter of 1.4 nm. The values for the A-Rh/TiO, catalysts fit well a raft-like particle model consisting of di- or tri-atomic Rh layers which should have Ni/Nl = 0.66 or 0.77, respectively. Similarly, A-Rh/Si02(473) is suggested to have a monolayer raft Rh structure from the particle size of 1.3 nm and N i / N , = 0.42 0.17.The same conclusion is valid also for A-Rh/Si02(773), indicating the stability of this structure attached on SiO,. These structures are compatible with the chemisorption data. Fig. 7 shows the near-edge structure of the Rh K-absorption spectra of Rh metal and A-Rh/Ti02(473). The absorption edge of the A-Rh/TiO, catalyst shifted towards higher energy compared to that of Rh metal. The reproducibility of this shift was confirmed by two independent X-ray absorption measurements. Table 3 shows the energy shifts of the Rh K-absorption edge of A-Rh/TiO,, I-Rh/TiO, and A-Rh/SiO, referred to the edge energy of Rh metal, where the edge energies are taken at the inflection point of the edge.The shifts of A-Rh/Ti02(473) and A-Rh/Ti02(773), 4.0 L- 0.3 and 3.1 f 0.3 eV, respectively, are quite large compared with 0.4 & 0.3 eV of I-Rh/Ti02(473) and (773), while A-Rh/SiO, showed no edge shift. There are two possibilities for the cause of the energy shift; one is the reduction of relaxation energy arising from a decrease of the particle size and the other is the large positive charge of the Rh particles. The edge energy of A-Rh/SiO, was the same as that of Rh metal (edge shift = 0.0 eV) although the Rh particle size of the catalyst (1.3 nm) was as small as that of A-Rh/TiO, (1.4 nm). Furthermore, the particle sizes of the I-Rh/TiO, catalysts (3.4 nm) were much larger than those of the A-Rh/TiO, catalysts as shown in fig.4. Therefore the change of the relaxation energy is unlikely to be important for the energy shift. Thus the positive charge of the Rh particles may be the primary cause of the positive edge shift. The Rh particles of A-Rh/TiO, are formed by H, reduction of the attached species B which have Rh-0-Ti bonds and are atomically dispersed at the TiO, surface, as shown by EXAFS. The controlled preparation procedure led to the formation of small Rh particles with a narrow size distribution. The small raft-like Rh particles would be stabilized partially due to the Rh-0-Ti bonds by which a positive charge is induced on the Rh assembly through electron transfer. It is quite difficult to estimate the correct amount of electron flow from the edge shift.However, we tried to guess roughly the charge of the Rh particles attached on the TiO, by comparing the edge shift with those of standard compounds. The energy shifts of Rh,0,(Rh3') and Rh,(CO),Cl,(Rh+) were observed to be 10.0+0.4 and 6.0f0.3 eV, respectively. The charge induced by the Rh-0-Ti bonding in the Rh particles may be well delocalized through the Rh-Rh metallic bonds. Therefore the oxidation number per Rh atom in the Ti0,-attached Rh particles is estimated to be < 1 + , i.e. the average number of Rh-0-(TiO,) bonds of an Rh atom is < 1. Because1338 New Ti0,-attached Rh Catalyst I ElkeV Fig. 7. Rh near-edge absorption spectra of Rh metal (--) and A-Rh/Ti02(773) (-).of this small coordination number, it was impossible to observe direct Rh-0 bonding from the analysis of EXAFS data. The edge energy of A-Rh/Ti02(773) was lower by 0.6eV than that of A-Rh/Ti02(473). This may be considered as a result of the contribution of the reverse electron transfer from Ti3+ produced by the high-temperature reduction, although the large positive shift of 3.1 eV is still observed as given in table 3. No edge shift was observed in A-Rh/SiO,, owing to the difference between A-Rh/SiO, and A-Rh/TiO, in the interaction of Rh and surface oxygen. The metal- oxygen interaction on SiO, is in general weak and possibly an ion (oxygen)-induced dipole interaction as Koningsberger and coworkers12 suggested. On the other hand, the Rh-0 bond in the Ti0,-attached Rh species must be a chemical bond which stabilizes the positive charge of the Rh particles.These Rh particles may be said to be ‘attached Rh metal clusters’ in the sense that the Rh assembly exists under a strong interaction with the surface oxygens as shown in fig. 8. Conventional Rh/TiO, catalysts nearly lose the ability to chemisorb H, and CO after high-temperature reduction (SMSI effect).’ Recent EXAFS studies on Rh/TiO, prepared by an ion-exchange method demonstrated that a direct Rh-Ti bond was produced in the SMSI state.14.15 The bond lengths reported by Haller et al.14 and Koningsberger et al.15 were 0.252 and 0.342 nm, respectively. The Rh-0 bond length given by Koningsberger et al. was 0.270 nm, similar to that in the Rh/A1,03 system.This means that the interaction between Rh and 0 is as weak as an ion-induced dipole interaction. We do not know why two reported Rh-Ti bond lengths were so different from each other, but it may be possible that these direct Rh-Ti bonds produce the SMSI state. Horlsey reported that the 0.7 electrons are transferred from Ti to Pt through the direct Pt-Ti bond on the basis of his Xa calculation^.^^ In contrast to the conventional catalysts, a ‘ Ti0,-attached Rh metal cluster ’ with a positively charged character shows no SMSI behaviour as has been mentioned above. The attached cationic Rh clusters had a dispersion of 50% (H/Rh = O.SO), which is much smaller than the H/Rh value expected from the particle size of 1.4 nm. Thus the adsorption of H, and CO does not show the extent of the metal dispersion in the charged Rh system. The ‘ Ti0,-attached Rh cluster’ catalyst which does not experience the SMSI phenomenon shows a high activity for the hydrogenolysis of ethane as compared with the conventional Rh/Ti02 catalyst, as shown in fig.9.7a The A-Rh/Ti02(773) catalyst showed a similar activity to the A-Rh/Ti02(473) catalyst, so the catalytic activity is capable of maintainingK. Asakura, Y. Iwasawa and H. Kuroda I 1.3nm Ir I I * I 1339 ( b ) t t o p Rh layer t s e c o n d Rh layer first Rh layer substrate 0 layer Fig. 8. A proposed structure of the attached Rh metal cluster on TiO,; (a) top view and (b) side view. reaction time/ min Fig. 9. Ethane hydrogenolysis on Rh/TiO, catalysts at 373 K. 0, species B (30 kJ mol-'); 0, A-Rh/Ti02(473) (51 kJ mol-l); x , A-Rh/Ti02(773) (53 kJ mol-l); -0, I-Rh/Ti02(473) (69 kJ mol-'); 0, I-Rh/Ti02(773) (82 kJ mol-'); ., A-Rh/Ti02(863) (132 kJ mol-').1340 New Ti0,-attached Rh Catalyst its high level under any reaction conditions as well as any pre-reduction temperatures.Our present work suggests that the non-SMSI behaviour of the ‘Ti0,-attached Rh clusters’ is due to the Rh-0-Ti bonds which were produced in the well defined attachment steps in scheme 1 and partially maintained after the high-temperature reduction. In summary, the Rh particle of the A-Rh/TiO, catalyst exists as small, thin metal clusters 1.4 nm large and with 2 or 3 Rh layers. The particular features of the attached Rh metal clusters such as the positively charged character, the shortening of the Rh-Rh distance and the thin raft-like structure are derived from the Rh-0-Ti bonds which were formed in the attachment procedure of the Rh-ally1 complex and partially retained after high-temperature H, reduction.Such Rh-0-Ti bonds cause the characteristic behaviour in the chemisorption of H, and CO and the high activity for ethane hydrogenolysis. We are grateful to Dr M. Numura and the PF staff for their technical help in the EXAFS measurements. References 1 Y. I. Yermakov, B. N. Kuzunetsov and V. A. Zakharov, Catalysis by Supported Complexes (Elsevier, 2 Tailored Metal Catalysts, ed. Y. Iwasawa (D. Reidel, Dordrecht, 1986). 3 Y. I. Yermakov, B. N. Kuzunetsov, L. G. Karakchiev and S. S. Derbeneva, Kinet. Katal., 1973, 14, 4 G. Carturan, G.Facchin, G. Cocco, S. Enzo and G. Navazio, J. Catal., 1982, 76, 405. 5 M. D. Ward and J. Schwarz, J. Am. Chem. SOC., 1981, 103, 5253. 6 H. C. Foley, S. J. DeCanio, K. J. Chao, H. H. Onuferko, C. Dybnowski and B. C. Gates, J. Am. 7 (a) Y . Iwasawa and Y. Sato, Chem. Lett., 1985, 507; (b) M. D. Ward and J. Schwartz, J. Mol. Catal., 8 S. J. Tauster, J. C. Fung and R. L. Garten, J. Am. Chem. SOC., 1978, 100, 170. 9 D. E. Resasco and G. L. Haller, Metal-Support and Metal-Additive Effects in Catalysis, ed. B. Imelik, C. Naccache, G. Coudurier, H. Praliaud, P. MCriaudeau, P. Gallezot, G. A. Martin and J. C. Vedrine (Elsevier, Amsterdam, 1982). Amsterdam, 1980). 709. Chem. SOC., 1983, 105, 3074. 1981, 11, 397. 10 F. W. Lytle, G. F. Via and J. H. Sinfelt, J. Chem. Phys., 1977, 67, 3831. 11 K. Asakura, M. Yamada, Y. Iwasawa and H. Kuroda, Chem. Lett., 1985, 511. 12 J. B. A. D. Van Zon, D. C. Koningsberger, H. F. J. Van’t Blik, R. Prins and D. E. Sayers, J. Chem. 13 P. Legarde, T. Murata, G. Vlaic, E. Freund, H. Dexpert and J. P. Bournonville, J. Catal., 1984, 84, 14 S. Sakellson, M. McMillan and G. L. Haller, J. Phys. Chem., 1986, 90, 1733. 15 D. C. Koningsberger, J. H. A. Martens, R. Prins, D. R. Short and D. E. Sayers, J. Chem. Phys., 1986, 16 H. Oyanagi, T. Matsushita, M. Ito and H. Kuroda, KEK Report, 1984, 83-30. 17 Y. Iwasawa and K. Asakura, Homogeneous and Heterogeneous Catalysis, ed. Y. I. Yermakov and V. 18 B. K. Teo, Basic Principles and Data Analysis, Inorganic Chemistry Concepts 9 (Springer-Verlag, 19 B. K. Teo and P. A. Lee, J. Am. Chem. SOC., 1979, 101, 2815. 20 B. K. Teo, M. R. Antonio and B. A. Averill, J. Am. Chem. SOC., 1986, 105, 3751. 21 G. Pantini, P. Pacanelli, A. Immirzi and L. Porri, J. Organomet. Chem., 1971, C17, 33; DBM = 1,3- 22 R. B. Greegor and F. W. Lytle, J. Catal., 1980, 63, 476. 23 J. A. Horsley, J. Am. Chem. SOC., 1979, 101, 2870. Phys., 1984, 80, 3914. 333. 90,3047. Likholobov (VNU Science Press, Tokyo, 1986). Berlin, 1986). diphenylpropane- 1,3-dionato. Paper 7/084; Received 16th January, 1987

ISSN:0300-9599    

DOI:10.1039/F19888401329

出版商:RSC    

年代:1988

数据来源: RSC

摘要:

J. Chem. SOC., Faraday Trans. I , 1988, 84(5), 1341-1347 Hydrophilic and Hydrophobic Phenomena in Electrolyte Solutions Chai-fu Pan Department of Chemistry, Alabama State University, Montgomery, Alabama 361 95, U. S. A . The hydration parameter values of alkali-metal chlorides, bromides, iodides and nitrates at 25 "C have been studied from the extrapolation of activity coefficient data. The ion-size parameter H used in the simplified form of the Stokes and Robinson equation is chosen as the sum of Marcus's aqueous ionic radii of the cation and anion of the electrolyte. Values of activity coefficients are taken from Robinson and Stokes. Caesium chloride, caesium bromide, caesium iodide, potassium nitrate, rubidium nitrate and caesium nitrate show negative hydration, while other salts show positive hydration.Information about hydration effects in aqueous electrolyte solutions can be obtained from vapour pressure or activity coefficient data.'. These effects are usually described quantitatively in terms of the so-called 'hydration numbers ' or 'hydration parameters '. Various methods and interpretations may result in different values of the hydration number for an electrolyte. Conway3 has given detailed surveys of the theories and methods that lead to different results. The significance of the hydration parameter in the Stokes and Robinson model of hydration has been studied recently.' However, electrolytes with negative hydration parameters have not been generally investigated or discussed. The phenomena of hydrophilic hydration (positive hydration) and hydro- phobic hydration (negative hydration) are of interest and great significance in the investigation of structure of aqueous solutions. They correspond to the structure- promoting and structure-breaking effects of ions in water.The salt-in or salt-out effect and the exothermic or endothermic solution process may also be explained in terms of these phenomena. This study is limited to some uni-univalent electrolytes, including alkali-metal chlorides, bromides, iodides and nitrates. The values of hydration parameters in each system ranged from positive to negative. Aqueous Ionic Radii Stokes and Robinson4 proposed a model of hydration to describe the ion-water interactions in aqueous electrolyte solutions. In dilute regions, their equation for 1 : 1 electrolytes reduces to a simpler f0rm~9~ = lnfk,DH+0.036(h-l)m (1) where in which y+ is the mean molal activity coefficient, f+,DH is the mean rational activity coefficient calculated from the Debye-Huckel equation, h is the hydration parameter, Kl and K, are constants and m is the molality of the solution. For aqueous sol$ons at 25 "C, Kl = - 1.17604, K, = 0.328618 4, where h is the ion-size parameter in A.13411342 Hydrophilic and Hydrophobic Phenomena Table 1. Stokes-Robinson hydration parameter values of alkali-metal chlorides at 25 OCa LiCl NaCl KCl RbCl CsCl m y+ h y* h ?* h y+ h Yk h 0.1 0.790 17.5 0.778 11.2 0.770 6.11 0.764 3.09 0.756 -1.06 0.2 0.757 15.7 0.735 9.94 0.718 4.82 0.709 2.38 0.694 -1.59 0.3 0.744 14.7 0.710 8.90 0.688 4.35 0.675 1.98 0.656 -1.53 0.4 0.740 14.0 0.693 8.14 0.666 3.91 0.652 1.90 0.628 -1.48 0.5 0.739 13.4 0.681 7.61 0.649 3.59 0.634 1.80 0.606 -1.41 0.6 0.743 13.0 0.673 7.25 0.637 3.46 0.620 1.76 0.589 -1.26 a Values of d (A): LiCl (2.51), NaCl (2.81), KCI (3.17), RbCl (3.31), CsCl (3.52).Table 2. Stokes-Robinson hydration parameter values of alkali-metal bromides at 25 "CU LiBr NaBr y* m 0.1 0.796 18.8 0.2 0.766 16.7 0.3 0.756 15.6 0.4 0.752 14.7 0.5 0.753 14.0 0.6 0.758 13.5 0.782 12.0 0.741 10.5 0.719 9.55 0.704 8.77 0.697 8.47 0.692 8.15 KBr y+ h 0.772 6.16 0.722 5.05 0.693 4.55 0.673 4.21 0.657 3.88 0.646 3.75 RbBr CsBr 0.763 2.08 0.706 1.26 0.673 1.25 0.650 1.28 0.632 1.25 0.617 1.19 0.754 -2.42 0.694 -2.10 0.654 -2.25 0.626 -2.09 0.603 -2.04 0.586 - 1.82 a Values of 6 (A): LiBr (2.62), NaBr (2.92), KBr (3.28), RbBr (3.42), CsBr (3.63).Table 3. Stokes-Robinson hydration parameter values of alkali-metal iodides at 25 O C a LiI NaI KI RbI CSI y+ h y* h Y? h y+ h y* m 0.1 0.815 23.6 0.787 12.0 0.778 6.68 0.762 0.11 0.754 -3.98 0.2 0.802 21.6 0.751 10.9 0.733 5.82 0.705 -0.23 0.692 -3.74 0.3 0.804 20.0 0.735 10.3 0.707 5.25 0.671 -0.14 0.651 -3.74 0.4 0.813 18.9 0.727 9.88 0.689 4.82 0.647 -0.04 0.621 -3.69 0.5 0.824 17.9 0.723 9.48 0.676 4.53 0.629 0.08 0.599 -3.27 0.6 0.838 17.1 0.723 9.23 0.667 4.38 0.614 0.14 0.581 -3.00 a Values of H (A): LiI (2.90), NaI (3.20), KI (3.56), RbI (3.70), CsI (3.91). Table 4. Stokes-Robinson hydration parameter values of alkali-metal nitrates at 25 OCa LiNO, NaNO, KNO, RbNO, CsNO, m y+ h y + h ?+ h y* h y* h 0.1 0.788 15.2 0.762 4.00 0.739 -6.68 0.734 -9.38 0.733 -10.9 0.2 0.752 13.5 0.703 2.55 0.663 -7.37 0.658 -9.08 0.655 -10.7 0.3 0.736 12.6 0.666 1.91 0.614 -7.16 0.606 -8.95 0.602 -10.4 0.4 0.728 11.9 0.638 1.44 0.576 -7.04 0.565 -8.89 0.561 -10.1 0.5 0.726 11.4 0.617 1.25 0.545 -6.91 0.534 -8.51 0.528 -9.79 0.6 0.727 11.1 0.599 1.05 0.519 -6.76 0.508 -8.17 0.501 -9.42 a Values of d (A): LiNO, (2.74), NaNO, (3.04), KNO, (3.40), RbNO, (3.54), CsNO, (3.75).C-F, Pan 1343 o c -I ' 0 0.1 0.2 0.3 0.4 0.5 0.6 m Fig.1. Stokes-Robinson hydration parameter values of alkali-metal chlorides at 25 "C : (a) LiCI, (b) NaCl, (c) KC1, ( d ) RbCl, (e) CsCl. A reliable value of the ion-size parameter h cannot be obtained independently. In general, its value is chosen to be greater than the sum of the crystallographic radii, but less than the sum of the hydrated radii of the cation and anion in the electrolyte.The Debye-Huckel theory deals with a solution containing ions of unspecific structure. All it needs to know is their size and charge. However, choosing the ion size or distance of closest approach is always a problem. Marcus? reported recently the aqueous ionic radii of 35 ions, obtained by diffraction and computer-simulation methods. His values for univalent ions, in general, are slightly greater than Pauling's ionic radii.' Accordingly, Marcus's aqueous ionic radii should be a better source of ion sizes in aqueous solutions. Marcus's values of aqueous ionic radii for the corre2ponding ion? in solution $udied in thisD article are as follows : Li+ 10.68 A), Na+ (($98 A), K+ (1.34 A), Cs+ (1.69 A), C1- (1.83 A), Br- (1.94 A), I- (2.22 A), PO, (2.06 A).The value of the ionic radius for Rb+ is not available. A value of 1.48 A for Rb+ is chosen from Pauling.' The mean molal activity coefficients for these electrolytes in the concentration range 0.1-0.6 mol kg-l are taken from Robinson and stoke^,^ and are presented in tables 1-4. The values of h calculated from eqn (1) at the corresponding concentrations are also presented in these tables. Plots of h us. m are shown in fig. 1-4. In infinitely dilute solutions the phenomenon of hydration exists and the hydrated ions should follow the Debye-Huckel law exactly. Therefore, the limiting values of h on the plots are significant hydration parameter .values in the Stokes and Robinson model of hydration.2 The extrapolation procedure is necessary ; otherwise other deviations from the Debye-Huckel law1344 Hydrophilic and Hydrophobic Phenomena h I I -\ i Fig.2. Stokes- -R -4 * o 0.1 0.2 0.3 0.4 0.5 0.6 rn (b) NaBr, (c) KBr, (d) RbBr, (e) CsBr. .obinson hydration parameter values of alkali-metal bromides at 25 "C: (a) LiBr, contribute, in part, to the derived value of the hydration parameter. It is particularly important for the nitrates studied in this article as ion association occurs even in their dilute solutions. lo Negative Hydration Phenomenon The phenomenon of negative hydration has been mentioned by Robinson and stoke^.^ Potassium nitrate and potassium chlorate were cited by Sugden as examples.11 Sugden's paper was probably the first to draw attention to the structure of water as an additional factor in the problem of hydration.He explained that the negative hydration is a result of a depolymerizing effect of the salts containing large anions on the structure of water. Conway3 states that hydrophobic interactions in aqueous ionic solutions arise when functional groups on a charge centre have less tendency to interact with water molecules than do water molecules amongst themselves. For example, tetra-alkyl- ammonium salts, alkylamine ions and alkyl or aryl sulphonates show hydrophobic interactions with water. Samoilov12 places special emphasis on the negative hydration for simple ions. He believes that the existence of negative hydration shows directly that the concept of combination of water molecules with ions is not serviceable as a basis for a general treatment of hydration.Ionic hydration may be viewed as the effect of the ionsC- F. Pan 1345 26 1 1 I -6 + 0 0.1 0.2 0.3 0.4 0.5 0.6 m Fig. 3. Stokes-Robinson hydration parameter values of alkali-metal iodides at 25 “C : (a) LiI, (b) NaI, (c) KI, ( d ) RbI, (e) CsI. on the translational motion of the nearest water molecules, as well as interactions with the rest of the water. Many ions reduce the translational mobility of neighbouring water molecules, but some ions increase it. The latter phenomenon is considered to be the case for negative hydration. According to Samoilov, the alkali-metal cations, K+, Rb+ and Cs+, have negative hydration ; i.e.water molecules neighbouring such ions are more mobile than those in pure water. The central idea of the Stokes-Robinson model of hydration is that the real concentration of an ionic solution is greater than the stoichiometric concentration because some water molecules are tied to the ions and are not free to act as solvent. However, they neglect the phenomenon of negative hydration, in which some of the water is ‘released’ by the ions and is more free to move. Thus the hydration effect should be viewed as a result of the difference between water-water interactions in pure water, and ion-water and water-water interactions in solution. If the interactions in solution are stronger than those in pure water, positive hydration will result. Otherwise negative hydration will result.This study shows that some 1 : 1 electrolytes containing large cations or large anions, such as CsC1, CsBr, CsI, KNO,, RbNO, and CsNO, have negative hydration parameters, but LiCl, NaC1, KCl, RbC1, LiBr, NaBr, KBr, RbBr, LiI, NaI, KI, LiNO, and NaNO, show positive hydration. From the plot in fig. 3, it is obvious that RbI has almost ‘zero hydration’.1346 Hy droph ilic and Hydrophobic Phenomena 0 0.1 0.2 0.3 0.4 0.5 0.6 m Fig. 4. Stokes-Robinson hydration parameter values of alkali-metal nitrates at 25 "C : (a) LiNO,, (b) NaNO,, (c) KNO,, ( d ) RbNO,, (e) CsNO,. In a dissolution process, the heat of solution of a salt in water may be roughly estimated as the sum of the heat of fusion of the salt, the heat of ionization and the heat of hydration.The first two terms are always positive, while the third term, the heat of hydration, is negative for positive hydration and positive for negative hydration. Therefore, the heat of solution may be positive or negative if positive hydration is involved, but it can only be positive if it involves negative hydration. Heats of solution from literature1, infer that the electrolytes mentioned above showing negative or zero hydration all have positive values. They are (in cal? mol-'): CsCl (4250), CsBr (6210), RbI (6000), CsI (7970), KNO, (8340), RbNO, (8720), CsNO, (7560). Temperature affects hydration, but there is no consensus as to the sign of the temperature dependence. l4 For sodium chloride solutions, values of activity and osmotic coefficients are available in the temperature range up to 300 OC.15 These values suggest that hydration decreases with increasing temperature.Stokes14 believes this to be the case, as one expects heat to be evolved on hydration (positive hydration). If so, higher temperatures would favour negative hydration, since heat is absorbed in the process. Therefore, for hydrophobic electrolytes, such as those listed above, one would expect larger values of h at higher temperatures. t 1 cal = 4.184 J.C- F. Pan 1347 References 1 C. Pan, J. Phys. Chem., 1978, 82, 2699. 2 C. Pan, J. Phys. Chem., 1985, 89, 2777. 3 B. E. Conway, Ionic Hydration in Chemistry and Biophysics (Elsevier, Amsterdam, 1981). 4 R. H. Stokes and R. A. Robinson, J. Am. Chem. SOC., 1948, 70, 1870. 5 C. Pan, Can. J. Chem., 1976, 54, 9. 6 C. Pan, J. Chem. Eng. Data, 1977, 22, 234. 7 Y. Marcus, J. Solution Chem., 1983, 12, 271. 8 L. Pauling, The Nature of the Chemical Bond (Cornell University Press, Ithaca, NY, 1960), p. 514. 9 R. A. Robinson and R. H. Stokes, Electrolyte Solutions (Butterworths, London, 2nd edn, revised, 1965), pp. 5P-60, 491-495. 10 C. W. Davis, Ion Association (Butterworths, London, 1962). 11 J. N. Sugden, J. Chem. SOC., 1926, 129, 174. 12 0. Ya. Samoilov, Structure of Aqueous Electrolyte Solutions and the Hydration of Ions, transl. 13 Handbook of Chemistry and Physics, ed. R. C. Weast (Chemical Rubber Co., Cleveland, 53rd edn, 14 R. H. Stokes and R. A. Robinson, J. Solution Chem., 1973, 2, 173. 15 K. S. Pitzer and J. C. Peiper, J. Phys. Chem. Ref. Data, 1984, 13, 1. D. J. G. Tves (Consultants Bureau, New York, 1981), pp. 2, 94, 105-106, 170. 1972), p. D-78. 45 Paper 71179; Received 2nd February, 1987 FAR I

ISSN:0300-9599    

DOI:10.1039/F19888401341

出版商:RSC    

年代:1988

数据来源: RSC

摘要:

J . Chem. SOC., Furuduy Trans I, 1988, 84(5), 1349-1356 Thermal Decomposition of Copper@) Squarate Andrew K. Galwey,* M. Abdel-Aziz Mohamedt and Sundara Rajamz Chemistry Department, Queen 3 University, Belfast BT9 5AG Michael E. Brown Chemistry Department, Rhodes University, Grahamstown 6140, South Africa The thermal decomposition of copper(I1) squarate dihydrate, in vacuum between 530 and 670K, has been shown by kinetic and complementary analytical measurements to proceed (following rapid initial loss of the water of crystallization) through two reactions, involving stepwise reduction of the cation, Cu2+ -, CU' -+ Cuo. This pattern of behaviour has been described previously for the decompositions of several copper(~r) carboxylates. Isothermal fractional decomposition (a) us.time curves were deceleratory throughout and were analysed in two parts. The first stage of reaction (a < 0.5) was the decomposition of copper(I1) squarate to copper(r) squarate, studied in the temperature interval 530-590 K. The initial part of this rate process was zero-order with an activation energy of l50+ 15 kJ mol-l. Subsequently the rate diminished and reaction could be represented appi oximately by either a contracting-interface model or by one-dimen- sional advance of a reactant/product interface into flat crystallites of varying thicknesses. The second stage of reaction (a > 0.5) was the decomposition of copper(1) squarate to copper metal produced in the form of small crystallites dispersed on a carbonaceous matrix that was pseudomorphic with the reactant particles.This process, studied in the interval 590-670 K, obeyed the first-order equation approximately, when due allowance was made for overlap of the two regions, and the activation energy was 210+ 20 kJ mol-'. Electron microscopic observations of reac- tant, product and samples of partially decomposed salt did not reveal any recognizable reactant-product interface. It is suggested that Cu,O is formed as a decomposition intermediate, thus providing an explanation for the formation of product CO,. The observations are discussed and a reaction mechanism is proposed in the context of previous studies of the decompositions of copper(1r) carboxylates and of other metal salts of squaric acid. Recent st~diesl-~ of the thermal decompositions of various copper( 11) carboxylates have identified the common mechanistic feature that the reactions proceed through the stepwise reduction of the cation: CU2+ + Cu+ + CUO.This is a significant observation because it is often found that crystals containing common components exhibit marked differences in behaviour during decomposition. Accordingly, the identification of a unifying feature is of value in characterizing the factors that control reactivity. The recognition that the same intermediate participates in several reactions is invaluable in the formulation of reaction mechanisms for the various related rate processes. Kinetic, microscopic and analytical observations of the thermal breakdown of the following copper(r1) carboxylates5 have been published or are in the course of preparation for publication : copper(r1) formate,l copper(r1) t Permanent address : Chemistry Department, Assiut University, Qena, Egypt.1 Permanent address: QEM Government College for Women, Madras 600002, India 45-2 13491350 oxalate,2 copper(r1) mellitate,6 copper(I1) mal~nate,~ copper(r1) maleate4p and copper(I1) f~marate.~!' The present work was undertaken to extend the scope of this comparative study to include copper(n) squarate which contains carbon-oxygen-cation linkages that are different ftom those in the more extensively investigated carboxylates. In addition to extending this study of the copper(r1) salts, it was of interest also to investigate a further salt of squaric acid, H2C404. Few detailed studies of the decompositions of squarates have been published, but it has been shown that the kinetic characteristics of the decomposition of anhydrous silver squarate' are markedly different from those of nickel squarate dih~drate,~ where the removal of the stabilizing water of crystallization is an essential precursor step to anion breakdown.We are aware of only one previous kinetic study of the decomposition of copper(r1) squaratel' and this was a thermoanalytical investigation. The present work extends these studies by reporting microscopic observations, kinetic measurements based on gas evolution, and analytical measurements of the progress of cation reduction during reaction. Thermal Decomposition of Copper(1r) Squarate Experimental Preparation of Copper(I1) Squarate A 1 .OO mol dm-3 solution of squaric acid was mixed with an equal volume of 1 .OO mol dm-3 copper(I1) chloride and the solution was evaporated to half its volume.The solid product was removed by filtration and washed with warm distilled water, acetone and ether before being dried by evacuation for 48 h. This preparation was analysed by combustion analysis (C, H) and atomic absorption spectrometry (Cu). The composition was in very close agreement with that expected for the dihydrate, CuC404-2H,0. The mass loss on heating in vacuum at 460 K was consistent with the loss of two molecules of water appreciably below the decomposition temperature. Kinetic Measurements Kinetic studies were based on isothermal (& 1 K) measurements of the pressure of the gaseous products evolved in the constant volume of an initially evacuated ( Pa) glass apparatus.Two pressure gauges were used; a manual McLeod gauge and a Baratron diaphragm gauge. Output from the Baratron guage was recorded3 at appropriate time intervals and the measured values of pressure, time and temperature were stored in a Sinclair Spectrum microcomputer for later kinetic analysis. Gaseous products could be selectively condensed through use of a suitable cold-trap (78 or 175 K) maintained between the heated reactant and the gauge, thus providing a method of analysing the principal volatile compounds evolved (CO and CO,). Electron Microscopy Microscopic examinations were made using Jeol 35CF and JSM840 scanning electron microscopes. Samples of reactant, of product and of partially decomposed salt were precoated with a thin film of Au/Pd, except when using the back-scattering techniques in the JSM840 instrument.In spite of the coating, the samples tended to accumulate charge in the electron beam, thus reducing resolution. Internal structures of samples were revealed by gentle crushing after reaction but before coating. Results and Discussion Reaction Stoichiometry From measurements of the mass loss on completion of reaction (a = 1.00) and the gas pressures in the calibrated volume of the apparatus with either the 78 K (CO) or 175 KA . K. Galwey, M. A . Mohamed, S. Rajam and M. E. Brown 1351 .o a Fig. 1. The change in amount of Cu2+ present in the reactant (expressed as thiosulphate solution titration) with extent of decomposition (a). (CO + CO,) traps, the overall reaction stoichiometry under these conditions, can be satisfactorily expressed as CuC,O, * 2H20(s) + CU(S) + 3.03CO(g) + O.48CO2(g) + 0.5C(s) + 2H20.The X-ray powder diffraction pattern of the solid residue was that of copper metal. The water of crystallization was retained in the cold trap. The composition of the product gases was not appreciably influenced by the refrigerant used, showing that the residual solid did not catalyse the reaction 2co + CO, + c. These results are consistent with the previous thermochemical measurements. lo The en thalpy change reported for the oxidation of the residual product from decomposition in nitrogen was - 334 kJ mol-l, of which - 155 kJ mol-' was ascribed to the oxidation of metallic copper. The difference (- 179 kJ mol-l) may be attributed to the oxidation of the residual carbon to CO, (- 393.5 kJ mol-l) corresponding to (179/393.5) = 0.45 atoms, which is in very satisfactory agreement with 0.50 found above.Oxidation State of Copper Measurements of the changes of total amounts of Cu2+ ion remaining in the reactant during the progress of decomposition were made by volumetric analysis. Weighed samples of copper(r1) squarate were decomposed to various known extents, cooled and each was immediately dissolved in acidified KI solution. The iodine released was titrated with standard sodium thiosulphate. These measurements show (fig. 1) that the concentration of Cu2+ decreased linearly with a, becoming close to zero at a 2 0.5. Thus we conclude that reaction proceeds to completion in two distinct chemical steps: copper(I1) squarate -+ copper(1) squarate ( + gaseous products) -+ copper metal ( + gaseous products).1352 Thermal Decomposition of Copper( 11) Squarate This separation of the overall decomposition into two stages was not detected in the thermoanalytical study," but similar two-stage (Cu2+ -+ Cu+ -+ Cu') cation reduction has been observed in the decompositions of copper(r1) mal~nate,~ copper(I1) mellitate,6 copper(I1) maleate,4 copper(I1) fumarate4 and copper(I1) oxalate.[The absence of iodine release on dissolution of salts decomposed to a > 0.50 indicates that Cu+ does not disproportionate (2Cu+ -+ Cu2+ + Cu') during dissolution but precipitates as CuI. Iodine release is, therefore, accepted3 as a quantitative measurement of the amount of Cu2+ remaining in the reactant.] Electron Microscopy Samples of copper squarate examined included the initial copper(1r) squarate dihydrate reactant (a = O.OO), crystallites partially decomposed to various known values of a( k 0.02) and the final residual product (a = 1 .OO), as well as a portion of each sample which had been lightly crushed to reveal the internal structures.The prepared reactant salt was composed mainly of irregular aggregates of plate-like crystallites. A typical assemblage (a = 0.00) is shown in plate 1. Dimensions varied appreciably and shapes were often irregular. On (approximate) completion of the first reaction, a = 0.55, the dimensions and shapes of the crystals had not markedly changed, a typical texture is shown in plate 2.Decomposition resulted in the roughening of surfaces and crystal interiors (exposed by fracture, plate 3) developed an open froth-like texture. Continued reaction (a > 0.5) resulted in no marked change in crystallite appearance except for further roughening of surfaces, plate 4. It is concluded, from comparisons of these textures, and observations for other copper(I1) salts studied, that decomposition proceeds with the generation of an open, but coherent, intracrystalline matrix of residual carbonaceous material that preserves the shape of the reactant crystallites. The residual particles were pseudo- morphic with the original starting material. The copper metal produced possesses some mobility and the aggregation of Cu atoms into metallic particles, identified by the back-scattering measurements, explains the roughened surface textures3 that become evident as decomposition (a > 0.5) approaches completion. Clearly reaction does not involve general melting of the reactant.Whether or not there is restricted local and temporary fusion within individual particles could not be decided here since the internal textures were below the limits of resolution which could be achieved. Thus no information on the geometry of reaction development, or evidence on the presence of a reaction interface could be obtained. Kinetic Studies Kinetic measurements for the isothermal decomposition of copper(r1) squarate dihydrate were carried out in the temperature range 530-660 K. Studies were concerned with the prepared reactant, from which the water of crystallization was evolved (and condensed) prior to the onset of anion breakdown, unlike the behaviour of the nickel salt.g Plots of a us. time were deceleratory throughout with a small but significant reduction of the reaction rate at about the half-way stage, also characteristic of certain other copper(I1) carb~xylates.~? Consequently, no single rate equation from the group applicable to solid state decompositions".l2 was capable of representing the overall reaction. This behaviour can be satisfactorily explained by the analytical evidence that reaction proceeded by two consecutive rate processes. These are identified as the decomposition of copper(r1) squarate followed by the slower breakdown of copper(1) squarate. Analyses of the kinetic characteristics of these two reactions are discussed separately.J .Chern. SOC., Faraday Trans. I , vol. 84, part 5 Plates I and 2 Plate 1. A typical assemblage of unreacted crystallites of copper(I1) squarate dihydrate, representing the reactant salt as prepared (a = 0.00). Scale bar = 10 pm. Plate 2. Typical texture of crystallites of copper(I1) squarate decomposed to approximate completion of the first reaction (a = 0.55). Scale bar = 10 pm. A. K. Galwey et al. (Facing p . 1352)J. Chem. SOC., Faraday Trans. 1, vol. 84, part 5 Plates 3 and 4 Plate 3. The interior of a partially decomposed crystallite (a = 0.55), exposed by fracture. Scale bar = 10 pm. Plate 4. Crystallites reacted to completion (a = 1.00). Scale bar = 10 pm. A. K. Galwey et al.A .K. Galwey, M. A . Mohamed, S. Rajam and M. E. Brown 1353 0 10 20 LO 50 60 70 30 t/min Fig. 2. Plots of u us. time for the first reaction of copper(1r) squarate at different temperatures: (a) 532, (b) 550, ( c ) 566 and (6) 583 K. 0 5 10 15 20 25 30 35 LO t/min Fig. 3. Plots of u us. time for the decomposition of copper(I1) squarate at 563 K. An initial zero- order process is followed by a deceleratory process as the half-way stage is approached: (-) linear, (----) contracting area and ( a - . ) contracting volume. The First Reaction, Decomposition of Copper(I1) Squarate Typical isothermal a vs. time curves for this rate process are shown in fig. 2 for reactions at several temperatures within the range 532-583 K. Following an initially relatively rapid gas evolution (when a < 0.04), reaction proceeded at an approximately constant rate (zero-order kinetics) within the interval 0.04 < a < 0.32.Thereafter the process became deceleratory and merged, without appreciable discontinuity, into the second reaction, with which it overlapped. The gas yield corresponding to completion of the first reaction could not be measured, but the data acceptably obeyed both the contracting- area and the contracting-volume expressions1' (0.3 < a < 0.5). Kinetic characteristics1354 Thermal Decomposition of Copper(r1) Squarate using either 78 or 175 K traps were identical, confirming the conclusion that the residual solid did not interact with product gases. The initial zero-order behaviour, followed by the slightly deceleratory process during the approach to the half-way stage (fig.3), is very satisfactorily accounted for by an interface reaction proceeding inwards from the original planar surfaces of the predominantly flat and thin reactant platelets. This is one-dimensional growth, perhaps determined by the crystal lattice structure. The relative contributions towards the deceleratory character of the latter part of this reaction, from the variations in crystallite thickness and the inward movements of a reaction interface from edges in the more equidimensional crystals, cannot be decided from the evidence available here. Fig. 3 shows that the data conform to a contracting-envelope expression (assuming completion of the first reaction at a = 0.60). However, a similar curve could equally acceptably be derived, assuming one-dimensional growth and a reasonable distribution of crystallite thicknesses.The existence of a reaction interface could not be confirmed by electron microscopy, so any reaction mechanism formulated must be consistent with the analysis of the available kinetic data. The activation energy for the initial linear stage of the thermal decomposition of copper(I1) squarate was calculated as 150+ 15 kJ mol-1 from rate constants measured between 530 and 590 K, and log (Als-') was 10.5. The Second Reaction, Decomposition of Copper(1) Squarate The second reaction in the breakdown of copper(I1) squarate, identified as decomposition of copper(1) squarate, was slower than the first stage and was deceleratory throughout.Kinetic studies were completed in the interval 593-670 K, a temperature range appreciably higher than that in which rate studies for the first process were feasible. Data satisfactorily obeyed the first-order expression, usually in the range 0.75 < a < 0.93. At lower values of a (0.5 < a < 0.75) there is overlap with the final stages of the first reaction. The activation energy calculated was 210 +20 kJ mol-' and log (Als-') = 14.2. Comparative studies for copper(1) squarate (again3) required vacuum synthesis in situ by heating in the reaction vessel a crushed mixture of squaric acid with a stoichiometric excess of copper(1) oxide. After the rapid evolution of ca. 20 O h of the gaseous products, accompanied by some sublimation, there followed a slower rate process, identified as the decomposition of copper(1) squarate.This obeyed the first-order equation in the range 0.2 < a < 0.9, and two representative plots are shown in fig. 4. The value of the first-order rate constant for this reaction (0.0020 min-l at 592 K) is sufficiently close to values for the second stage of copper(r1) squarate decomposition (0.0031 min-' at 592 K) to confirm that the latter process is the breakdown of copper(1) squarate. There were no textural changes during the second stage of copper(I1) squarate decomposition, except for further roughening of crystal surfaces. We therefore conclude from the kinetic evidence that the rate is controlled by the first-order breakdown of individual molecules or of small crystallites in the reactant matrix.During such reactions the carbonaceous residue remains coherent and immobile, retaining the shape of the original particles, while the copper atoms aggregate to form metallic crystallites. An identical mechanism has recently been proposed3 to explain the similar behaviour of copper(I1) malonate. Reaction Mechanism In formulating a chemical mechanism for the decomposition of copper(^^) squarate, the significant features are the stepwise reduction of the cation and the evolution of 0.5 mol CO, per mol CuC,O, decomposed. CO, formation is identified as an essential feature ofA . K. Galwey, M. A . Mohamed, S. Rajam and M. E. Brown 0.6 h I - v Do -7 0.3 0 - 1355 - - I 1 I 0.9 I 0 salt breakdown because the maintained irreversible condensation of CO, during or after reaction did not result in CO removal.CO disproportionation is not promoted catalytically by the residue. The yield of CO, is most satisfactorily explained by the intervention of the lower oxide (0.5 mol Cu,O per mol salt) and its subsequent reduction by CO (+ OSCO,). This is an important similarity with the mechanism proposed for silver squarate decomposition* where, however, there is twice this proportion of oxide (-+ 1 .OCO,). There are common features in the mechanisms of decompositions of both salts in the scheme which we propose to explain the decomposition of copper(r1) squarate : 0 0 \c-c-0-cu-0-c-c // I II 11 I copper(I1) squarate c-c-0-cu-0-c-c 4 0 !?o 0 // I I 0 8 -c\, second reaction '0 I first reaction I 1 unstabie intermediate Cu20 [C,O,] unstable 1 intermediate 4CO CU20 i co + c + 2 c o lL 2 CU + COZ + C + 6CO Alternative reactant structures are also possible.We suggest that the first steps in decomposition are electron transfers, resulting in cation reduction, to yield copper(1)1356 Thermal Decomposition of Copper( 11) Squarate squarate and an unstable intermediate, [C,O,]. This reorganizes rapidly to carbon monoxide in greater yield (than from the silver salt) in this higher-temperature range, with the consequence that little or no oxygen is retained in the residual polymeric material, which is therefore amorphous carbon. The chemistry of decomposition of copper(r1) squarate shows several close relationships with the decompositions of copper(I1) mal~nate,~ copper(I1) maleate, and copper(r1) f~marate.~ The stepwise cation reduction provides a most satisfactory explanation of the marked diminution in rate observed at cc z 0.5.The copper(1) salt is relatively stable under high-temperature (anhydrous) reaction conditions. Parallels with the behaviour of silver squarate have been emphasized above. The intervention of the oxides has been suggested from the chemical similarities of the elements. Both yield a proportion of CO, equal to the amount of oxide assumed to be present as an intermediate. The reactions of both salts were predominantly deceleratory, exhibited no induction period, but occurred in different temperature ranges, possibly owing to the differences of electropositivity of the cations. The reactions of nickel squarate and of copper(I1) squarate differ in the relative ease of water rem~val,~ this may be due to the differences in crystal struct~re.'~ The stabilizing influence of lattice water is identified as controlling the kinetics of decomposition of the nickel salt.g Other obvious differences in the thermal behaviour of these compounds result from the intervention of the lower copper salt, since no parallel exists in the nickel compounds where reaction leads directly to product metal.The plate-like texture of the reactant crystallites, which is maintained throughout decomposition, is important in accounting for the slightly deceleratory kinetic behaviour observed. We thank Mr J. McCrae and his staff and Mr R. H. M. Cross for helpful advice in obtaining the electron micrographs. M. A. M. thanks the Egyptian Government and the ORS Award Scheme for Scholarships held during the period of this work. M.E.B. acknowledges financial support from the South African CSIR. 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Galwey, Comprehensive Chemical Kinetics, Vol. 22: Reactions in 12 D. A. Young, Decomposition of Solids (Pergamon, Oxford, 1966). 13 N. R. West and Y. Niu, J. Am. Chem. SOC., 1963, 85, 2589. the Solid State (Elsevier, Amsterdam, 1980). Paper 7/245; Received 10th February, 1987

ISSN:0300-9599    

DOI:10.1039/F19888401349

出版商:RSC    

年代:1988

数据来源: RSC